This study evaluated the effects of the dietary fish silage (FS) used as a replacement for fish meal (FM), on growth performance, fatty acid composition and serum variables of rainbow trout, Oncorhynchus mykiss. Four experimental diets, including 0% (FSO), 20% (FS20), 40% (FS40) and 60% (FS60) of fish silage, were formulated. Diets were given to triplicate groups of 40 fish/cage (average weight 52.90 +/- 1.88 g) for 8 weeks. The results showed that there were no significant differences in growth performance and nutrient utilization between control and FS20 group (p>0:05). Fish fed with the FS20 diet grew significantly better than others (p<0.05). Fillet protein and lipid content decreased with increasing of fish silage addition (p<0.05). FM replacement by FS did not affect the serum glucose level, but significantly increased total protein, triglyceride and total cholesterol levels (p<0.05). Fatty acid composition of fish fillet was no significantly different between groups (p>0.05). Results showed that fish processing by-product silage has potential to replace fish meal up to 20% in rainbow trout diets without adverse effects on growth performance, fatty acid composition and serum biochemical variables.