In the traditional system of medicine Ocimum sanctum Linn. is used to cure diseases like dysentery, bronchitis, malaria, bronchial asthma, diarrhea, arthritis, skin diseases, painful eye diseases, chronic fever. The essential oil obtained by using hydrodistillation of the leaves of Ocimum sanctum Linn. (purple variety) is collected throughout 1 y and analyzed employing gas chromatography equipped with flame ionization detector and gas chromatography connected with mass spectrometry. The yield of essential oil varied in the range from 0.14 to 0.21 % (w/w). 23 to 39 constituents were identified which represented 95.39±0.38 % of the total oil. The most abundant compound was identified as methyl eugenol with an average of 69.88±0.09 % with varying quantities from 55.48±0.36 to 76.78±0.54 %. The other constituents were beta-caryophyllene 10.40±0.06 %, alpha-ylangene 4.18±0.04 %, gamma-muurolene 3.64±0.04 % and borneol 2.02±0.01 %. The oil was found to be rich in phenylpropanoid (71.98±0.27 %) type constituents (average of 12 mo). A high amount of methyl eugenol (76.78±0.54 %) was observed during November while low (55.48±0.36 %) in April. It was noticed that when the plant was matured in April the accumulation of sesquiterpenes increased and phenylpropanoids decreased.