Tanacetum balsamita L. is a perennial rhizomatous plant of the Asteraceae family. T. balsamita is cultivated as medicinal, aromatic plants and food plant. Extract of costmary exhibits hepatoprotective, antiseptic and anthelmintic properties. The herb of T. balsamita contains essential oils and phenolic compounds. This species included in the "Biocollection of medicinal and aromatic plants of open and protected ground", Botanical garden of the VILAR. The main aim was to study the composition of phenolic compounds in the leaves of T. balsamita with application of ultra-performance liquid chromatography coupled with diode array detection and mass spectrometry. The object of the study was the leaves of T. balsamita. Samples were extracted with 1 ml of 80% aqueous acetone. An ultra-performance liquid chromatographic system with a photodiode detector and a triple quadrupole mass spectrometer was used for analysis of phenolic compounds. The UV and MS data of phenolic compounds were used for their identification or tentative characterization with application of mass spectrometry databases and data published in the literature.Results. The results obtained showed the presence in the leaves of 17 phenolic compounds. Five compounds were identified as caffeoylquinic, feruloylquinic and three dicaffeolquinic acid isomers, and two compounds as ferulic acid hexoside isomers. The leaves of T. balsamita contained also five luteolin derivatives, two chrysoeriol derivatives, and spinacetin/axillarin. Four compounds were not identified.