Abstract: The present research carried out to measure digestible (DE) and Metabolizable Energy (ME) for fishmeal and soybean meal in rainbow trout with two different sizes (120 and 220 g mean body weights). DE and ME are evaluation systems for feedstuffs and also, necessary for diet formulation. The fishes were maintained in metabolic chamber with single ingredient assay of feeding and chromic oxide (Cr2O3) was used as an external marker using total fecal collection method. Gross energy and chromic oxide values measured in feces, fishmeal and soybean meal. The results indicated that DE of fishmeal was 3700 and 3591 kcal kg-1 in 120 and 220 g body weight rainbow trout, respectively. Soybean meal had 3004 and 2889 kcal kg-1 respectively. Calculated ME from DE were 3204 and 3110 kcal kg-1 for fishmeal and 2601 and 2502 kcal kg-1 for soybean meal in 120 and 220 g rainbow trout, respectively. The results showed that rainbow trout can utilize fishmeal more efficiently than soybean meal.