Aromatic compounds in the steam distilled volatile oil obtained from aerial parts of Shrubby basil (Hindi - Ramtulasi), Ocimum gratissimum Linn. (Lamiaceae) were identified by gas chromatography (GC) and gas chromatography/mass spectrometry (GC-MS). The volatile oil mainly comprises of twenty (20) components, out of which all twenty constituents comprising 100 % of the oil were identified. The volatile oil mainly contains 13 monoterpenes (68.10 %) and bornyl acetate (14.5 %) was found to be predominant constituents followed by β-pinene (10.8 %), camphor (9.0 %), c-tagetone (8.6 %), t-tagetone (7.4 %), αpinene (4.2 %), myrtenal (4.1 %), α-thujene (3.0 %), α-camphene (1.8 %), O-methyl eugenol (1.6 %), ctagetone isomer (1.4 %), sabinene (0.9 %) and α-terpineol (0.8 %). Five sesquiterpene comprising (20.2 %) i.e. α-Selinene (5.9 %), t-caryophyllene (5.5 %), β-guaiene (4.0 %), β-gurjunene (2.0 %) and bicyclo germacrene (2.8 %). The two non terpenic components comprise (11.7 %) i.e. ethyl cyclohexenal ketone (7.1 %), n-butyl benzoate (4.6 %). The antimicrobial activity of O. gratissimum oil was studied by cup plate method against both Gram-positive and negative bacterial strains and pathogenic fungal species. Antimicrobial activity was concentration dependent on Shigella flexineri, Bacillus cereus and Candida albicans.