Plant galls caused by “insect” are unique example of host-pathogen interaction in which a complex association and mutual adaptation between the host and the pathogen takes place. Galls are characterized by cellular hypertrophy and hyperplasia. Eucalyptus tereticornis Smith, (Myrtaceae) is an important source of essential oil. Insect Leptocybe invasa infect stem, leaf midrib and petiole of Eucalyptus tereticornis which results into the gall formation. The present investigation deals with the evaluation of phytochemical compounds present in the normal leaf and the alteration that is observed in the midrib gall and petiole gall on Eucalyptus tereticornis through Gas Chromatography-Mass Spectroscopy technique. Methanolic extract of plant samples were used for the study of phytochemical constituents. GC-MS analysis of normal leaf, midrib gall and petiole gall of Eucalyptus tereticornis showed the presence of sixty six, thirty nine and forty four phytochemical compounds respectively. Further investigation on the effectiveness of these phytochemical constituents can result into various findings and discovery of novel and useful secondary metabolites which can be used as various purposes in medicinal and other related industries.