The essential oil is a main product of Holy basil (Ocimum sanctum L.). It present mainly in the peltate glandular trichomes. Therefore, the morphological and physiological changes during the development of peltate glandular trichomes are of great interest. In this paper, the morphological changes of glandular trichomes, the relationship between glandular trichomes and leaf development, and the physiological changes during leaf development of Holy basil were analyzed. The Holy basil peltate glandular trichomes were formed at the early stage of leaf development (leaf primordia) and continuously renewed during leaf development. The development of peltate glandular trichomes of Holy basil included four main stages: initiation, shaping, secretory cavity formation and essential oils accumulation. The formation of peltate glandular trichomes started at the beginning of leaf rapid growth while the accumulation of essential oils occurred at the slow mature growth and strongly increased at the stationary growth phase. The intensity of photosynthesis and respiration of leaves increased strongly during the period of rapid growth. The activities of auxins and cytokinins in leaf were very high at the beginning of the rapid growth stage while gibberellin activity was high only at the end of the rapid growth stage. Treatment with 1 mg/L gibberellic acid increased the length of branch, area of leaf, and the content of the essential oil.