Lagerstroemia speciosa (L.) Pers (Hindi - Jarul, Taglog - Banaba) a member of Lythraceae family is found all over India, specially in Bengal, Assam and Deccan Peninsula. The leaves of L. speciosa is widely used for lowering blood sugar levels in Philippines, Japan and Taiwan. Pharmacognostical investigation of leaves of L. speciosa was done by evaluating its morphological, microscopical studies, Physicochemical and phytochemical parameters. Rasa Nirdharana was done. Microscopical study revealed presence of epidermal cells contained clusters of rosette aggregate calcium oxalate crystals and few cells were mucilaginous. Lower epidermis showed the presence of anomocytic stomata. Physical constants of leaf powder showed, loss on drying - 3.8%, total ash - 6.7%, acid insoluble ash - 1.039%, water soluble extractive value - 8.88%, alcohol soluble extractive value - 4.49% and pH - 5.75. Preliminary phytochemical study revealed the presence of alkaloids, flavonoids, saponins, triterpenes, tannins, proteins and iron. Rasa Nirdharana confirmed the leaves are Kashaya in Rasa. The Pharmacognostical study was useful for authentication of leaves of Lagerstoemia speciosa.