In order to study the effects of partial replacement of cottonseed meal for fish meal on rainbow trout (Oncorhynchus mykiss) performance and also effect of Ferosulfate to neutralize gossypol poisonousness in cottonseed meal, 7 replacement levels (0, 10, 20, 25, 30, 35 and 40 percent) of meal with and without Ferosulfate were considered and compared at a basic design of completely randomized by factorial experiment (2´7). 840 pcs fish the average body weight of fish were100±5 gr, distributed equally in 42 pens and experiment was continued for ten weeks. Body weight and condition factor (CF) were measured every 2 weeks after 2 weeks of acclimatization. Feed cost per a kilo of fish producing, protein efficiency ratio (PER) and apparent net protein utilization (ANPU) were measured at the end of trial. No mortality resulting of experimental treatment was observed during the trial. The results showed that there was no significant difference between results of replacement of cottonseed meal for fish meal at the same levels (specially %20) and control diet (without cottonseed) (p>0.05). After control diet, %20 replacement had the best results. It is probably due to suitable balance of essential amino acids in the ration. In general, adding the Ferosulfate to cottonseed meal diets caused no significant improvement in fish performance .This results showed that the level of gossypol in cottonseed meal used was low. Of course addition of Ferosulfate with high levels of cottonseed meal had desired controlling effect on gossypol. However, high levels of cottonseed meal (specially %40) caused more significant decrease on attributes measured than control diet .According to these results, it seems that lower efficiency of diets high in cottonseed meal isn’t completely result of undesired effects of gossypol. Other reasons for low efficiency of diet high in cottonseed meal in this experiment may be due to the followings: lack of balance between essential amino acid, specially lysine, decreased palatability and subsequently decreased fish appetite, and as a result wasting of some received dietary and increased dietary crud fiber content.