The thesis comprises two parts, A and B. Part A deals with the analysis and quantification of medicinally important essential oil, by mass spectrometry method while, the part B describes the chemical studies on cladode of Opuntia dillenii. A short summary on the biosynthesis of steroids, flavonoids, fatty acids and terpenoids are also included in the thesis. Part A This part explains the chemical constituents and their quantification of medicinally important components of extracts on Ixora fulgens, I. coccinea (yellow and orange color flowers), I. polyantha, I. chinensis, Ipomoea batata (blackie), I. batata pink frost, and Cassia fistula, by mass spectrometry method. GC and GC-MS studies were carried out on extracts of different parts especially flowers, leaves and stalks of the plants which led to the identification of 230 chemical compounds from hexane extracts, 324 chemical compounds were identified from chloroform extract of Ixora species. Moreover, 26 metabolites were characterized from methanol extracts of Ixora chinensis flowers extract. From Ipomoea species, 106 chemical compounds were identified from hexane extracts, while, 72 compounds were identified from chloroform extract of the plant. Moreover, 57 phytochemicals were identified from hexane extract, whereas, 51 metabolites were identified from chloroform extracts of C. fistula plant. Different class of compounds were identified from this extracts including hydrocarbons, long chain fatty acids and their esters, alcohols, aromatic acids and their esters, terpenes and vitamins. Antibacterial, antifungal, antioxidant, anti-inflammatory, and anti-cancer activities have also been carried out from all the above mentioned extracts. Part B Total thirteen different fractions of vaccum liquid chromatography (VLC) of antidepressant butanol phase of Opuntia dillenii were selected for chemical analysis. For this purpose GC and GC-MS studies were carried out, which led to the identification of 409 chemical compounds.