The main goal of the current study was to investigate the effects of low protein diets along with fishmeal replacement by plant protein in rainbow trout diet. In the current study, three protein levels (32, 36 and 40%) and three fishmeal levels (20, 40 and 60%) were tested in rainbow trout. Amino acid content of the diets were balanced by addition of essential amino acids. This led to nine experimental diets and three replicates for each treatment. The results showed that growth parameters were influenced by protein level (p<0.05). However, fishmeal replacement only changed final weight and specific growth weight (p<0.05). The result also revealed that the interaction effect between protein level and fishmeal replacement was significant only for specific growth rate (p<0.05) and there were no interaction effect for other growth parameters (p>0.05). Maximum growth was observed in fish fed diet containing 36% protein and 40% fishmeal. Both protein level and fishmeal replacement affected TAN excretion in rainbow trout (p<0.05) and minimum ammonia nitrogen was observed in treatments 36p20f and 32p40f (p<0.05). In conclusion, the current result suggested that lowering dietary protein by 36% and also fishmeal replacement by plant protein up to 60% did not have a negative impact on growth parameters in rainbow trout when dietary essential amino acids were balanced. This condition also can lead to a lower ammonia release to water.