Nigella sativa and N. damascena are major spices and aromatic plants that are cultivated commonly in Turkey. The fatty acid compositions of full maturated seeds in the species were compared in the research. Variations in the fatty acid composition of the oil in the species were also examined during the four maturation stages of seeds, immature, premature, mature, and delayed mature. Quantitative variation in the fatty acid compositions were significantly (p < 0.01) different between the species. Linoleic and palmitic acid contents were higher statistically (p < 0.01) in N. sativa with 57.0 and 12.5 %, respectively while the oleic acid content was higher in N. damascena with 32.2 %. During the maturation stages, linoleic acid - the main fatty acid in both species - contents decreased regularly from 57 to 51 % during maturation stages in both species. On the other hand, the oleic acid amount (the main saturated fatty acid in both species), increased from immature stages to delayed mature stages and were found to be 20.0 and 27.2 %, respectively. According to the results, it can be concluded that harvesting from fully mature stages is preferable than other stages. This is due not only to the significant difference in fatty acid content of immature and premature stages but also the harvesting loss of the delayed maturation stages.