The article is honored the personality of famous Ukrainian and Russian historian, scientist, publicist, romantic poet, thinker and public figure, one of the leaders of Cyril and Methodius Society, an archeographer, professor of St. Volodymyr University Mykola Kostomarov (1817–1885). The urgency of the study is to honor Mykola Kostomarov through the researching the period of his formation as a university lecturer and a researcher during his work at St. Volodymyr University. It is considered his activities as a university lecturer and a researcher at St. Volodymyr University from August 1846 till March 1847 during Kyiv period (1844–1847) of his life. There were analyzed Mykola Kostomarov’s memories, which he wrote in his autobiography, asvaluable sources for the study of Mykola Kostomarov as a university lecturer and a researcher at St. Volodymyr University. He remembered his colleagues and students, described the atmosphere of the University in the whole. It is examined the contents and significance of ethnographic work “Slavyanskaya Mythology”, which became a part of the lectures which he taught at St. Volodymyr University during the second half of 1846. It is noted that scientist explored the poetic views of ancestors of ancient Slavs on the nature and their perception of the world and mythology in “Slavyanskaya Mythology”. Mykola Kostomarov depicted the contents of Slavs mythology in the first part of the work. He described holidays of the Slavs-pagans in the second part and presented his thoughts the about service of the Slavs-pagans in the third part. It is emphasized that Mykola Kostomarov wrote “Slavyanskaya Mythology” on fragments of antiquity, which were preserved in folk legends, legends, fairy tales, spells, songs, beliefs, and were researched by written sources. According to Mykola Kostomarov, Slavonic peoples already had in the pagan period of their life a well-developed system of higher mythology, considerable remnants of which were preserved in later folklore of Ukrainians and Russians. Mykola Kostomarov scientifically verified this thesis, outlining his task as a research of unified system of mythology of Slavic peoples, the basis of which he had seen in worship of Sun and stars. It is emphasized that Mykola Kostomarov scientific research was in keeping with the European ethnography and folklore of the end of 18th – the middle of 19th centuries. His interest in the Slavic mythology took place in an atmosphere of enlightenment and romantic attention to the folk life and folk poetry. “Slavyanskaya Mythology” became an important step in the study of mythology in Ukraine and Russia. It is emphasized that the significance of Mykola Kostomarov scientific heritage in the development of Ukrainian and Russian historiography is very important. Mykola Kostomarov was the first scientist in the Eastern Europe, who changed the approach to the work of historian drastically. Mykola Kostomarov put the history of people in its social cultural integrity and unity of different spheres of life as the main aim of historian’s work, instead of description of events and persons. Thus, Mykola Kostomarov founded a populist direction of historiography — the populist school, the main focus of which was aimed at highlighting the history of the masses as the leading driving force of social progress. It is noted the activity of Mykola Kostomarov in the Kyiv Archeographic Commission in preparation for the publication the Chronicle of Samiylo Velychko, the most monumental Ukrainian historical and memoir prose of XVII–XVIII centuries, which together with Chronicle of Samovydets, Chronicle of Gregory Hrabianka, and “History of Rus’” formed a complex of Cossack historiography. It is analyzed memories of Mykola Kostomarov from his autobiography about his colleagues, students, and the existing atmosphere at the University in that time. It is marked that Mykola Kostomarov’s scientific achievements were commended by St. Volodymyr University. It recognized him as a doctor of history in 1864 and chosen him as an honorary member in 1884. It is proved that Mykola Ivanovych Kostomarov occupies one of the most honorable places among the representatives of the School of historians of St. Volodymyr University. It is proved that his formation as a university lecturer and a researcher, and the direction of his whole life as a citizen, a historian and a scientist were formed during his work at St. Volodymyr University. Mykola Kostomarov considered himself the Kyiv period of his life and work at St. Volodymyr University as the happiest and brightest moments of his life.