Abstract: A 90-day feeding trial was conducted in indoor culture system to evaluate the effects of replacing fish meal with phytase- pretreated soybean protein isolate (PP-SPI) in feeds for juvenile Rainbow trout (Oncorhynchus mykiss). The fish initial body weight (4.86?.02) g were fed six isonitrogenous and isocaloric feeds (crude protein, 44.97%;crude lipid, 13.42%). The diets were formulated to produce diets in which 0, 200, 400, 600, 800 and 1000 g/kg of protein from fish meal were replaced with that from PP-SPI. SPI was pretreated by phytase at the level of 1000 U/kg. The results showed that the specific growth rate (SGR) increased significantly when the PP-SPI replacement level from 0 to 40%, and there was a significant decrease for PP-SPI replacement level from 40% to 100%. Weight gain and protein efficiency ratio (PER) showed the same trend as in the SGR. The minimum value (1.10) of FCR was at 40% replacement (S40) of fish meal with PP-SPI (P<0.05), and there was a significant increase for PP-SPI replacement level from 40% to 100%. P excretion significantly decreased as the proportion of PP-SPI replacement in the diet increased. However, efficiency of P retention significantly increased with increasing PP-SPI level (P<0.05). The N excretion decreased with increase of PP-SPI replacement level from 0% to 40%, then increased with the increase of PP-SPI replacement level from 40% to 100%. The N retention increased with increasing levels of PP-SPI replacement to 40%, while decreased with increasing levels of PP-SPI replacement from 40% to 100%. The results of the present study showed that 40% of fish meal could be replaced by PP-SPI in diets of juvenile rainbow trout with maximum weight gain or feed efficiency.