The activity of three C18 fatty acids (oleic, stearic and linoleic acids) was conducted under laboratory conditions on 1st instar larvae of spiny bollworm, Earias insulana (Biosd.). The results showed that oleic and stearic acid were more effective than linoleic acid with different concentrations. Results of the latent effect of LC50 concentrations for the three tested fatty acids showed that all the acids elongate the larval & pupal period than control of SBW. Furthermore, stearic acid introduce the shortest larval and pupal duration (18.93days and10.19 days, respectively) than the other two tested acids. Studying biochemical and enzymatic activities in the resulted 4th instar larvae of E. insulana after treatment with the LC50 concentration cleared that linoleic acid recorded increasing in total protein content (46.93%). In contrast, oleic and stearic acid showed reduction (-28.49&-1.96, respectively). Total carbohydrates and total lipids contents decreased in the three treatments compared to the control. The highest decrease in carbohydrates content recorded in oleic acid treatment (-31.37%) while, stearic acid recorded the highest decrease in total lipids content (-35.29%). Also, enzymes activity cleared that Alkaline phosphatase (ALP) increased in the oleic, stearic and linoleic acids treatments (33.23, 108.6 and 51.24%, respectively) and Glutamic oxaloacetic transminase (GOT) activity was increased by 22.35% in stearic acid treatment ; while it decreased in case of oleic and linoleic acids treatments (-35.53 and -23.71%, respectively). Glutamic pyruvic transminase (GPT) was decreased (-38.59%) in the stearic acid treatment while it increased slightly in oleic and linoleic acids treatments (8.48 & 12.79%, respectively) when compared with control