In the present study, fatty acid composition was determined in eyed egg embryo and larval development stages in Rainbow trout (Oncorhynchus mykiss). All fatty acids revealed significant differences between eyed egg and 10-days larvae (p<0.05) except for docosahexaenoic acid (DHA)/eicosapentaenoic acid (EPA), eicosapentaenoic acid (EPA)/docosahexaenoic acid (DHA), total unsaturated fatty acids (∑ USFA)/ sum of unsaturated fatty acids (∑ SFA). However, in both larvae before (19, 22, 24, 27, 29) and after complete absorption of the yolk sac (34, 39, 44, 49, 58), there was an apparent preference in the utilization of polyunsaturated fatty acids (PUFA) followed by monounsaturated fatty acids (MUFA). Palmitic acid (C16:0), oleic acid (C18: 1n-9) and DHA (C22: 6n-3) were the main SFA, MUFA and PUFA, respectively, which were utilized and significantly decreased (p< 0.01) from 12-day larvae (day 19, with 80% yolk sac) to 22-day larvae as well as from 27-day larvae to 51-day larvae and then reached to the minimum levels in 22 and 51-day larvae. In larvae, these 3 nutrients were also the most fatty acids utilizing as energy source and possibly as precursors for others monounsaturated fatty acids biosynthesis as well. During the same period and among (n-6) polyunsaturated fatty acids, linoleic (18:2n-6); LA) and arachidonic (20:4n-6); AA) acids contents significantly decreased (p< 0:05) from 12-day larvae to 22-day larvae as well as from 27-day larvae to 51-day larvae. DHA is generally spared for physiological functions.