The essential oils were isolated from Thymus serpyllum L. leaves, stem, flowers and whole plant using hydrodistillation technique and % yield was determined. The chemical constituents were studied using Gas Chromatography-Mass Spectrometry and Gas Chromatography-Flame Ionisation Detection. The analysis revealed the presence of 38 compounds in the essential oils obtained from different plant parts. Identification of compounds was achieved by detailed analysis of mass spectrum and retention indices. The major constituents of essential oil from the whole plant were thymol (35.25%), p-cymene (27.57%), thymol acetate (8.03%) and Î³-terpinene (6.92%). In leaves they were thymol (45.59%), p-cymene (24.26%), thymol acetate (6.41%), and Î²-bisabolene (5.91%); whereas in flowers these were thymol (68.74%) and thymol acetate (7.40%) and in the stem, these were thymol (55.15%) and thymol acetate (8.01%). Specifically, it is the first report wherein essential oil constituents of T. serpyllum L. whole plant, leaves, flowers, and stems have been analyzed separately. Further, fundamental analysis of complex mass fragmentation patterns of terpenes and terpenoids using EI-MS has been achieved and the logical reason for the fragmentation pattern has been elaborated. Detailed elaboration of the mass fragmentation pattern may strengthen the EI-MS-based compound identification of terpenes and terpenoids from essential oils.