An 8week laboratory trial was designed to study the effect of phytase on growth, body composition and phosphorus content in tissues of juvenile grass carp (〖WTBX〗C. idellus〖WTBZ〗) fed plant protein based diet. Seven trial diets were formulated to feed fish (with mean initial body weight of 39.68±3.05 g) that were divided into 5 experimental groups and 2 control groups. Five triplicate experimental groups were supplemented with five different levels of phytase (500 U/kg, 750 U/kg, 1 000 U/kg, 1 250 U/kg, 2 500 U/kg). The positive control group was supplemented with 1.5% monobasic calcium phosphate; and the negative control group was not supplemented with phytase and monobasic calcium phosphate. Through 60 d experiment,the effects of phytase were examind on growth, body composition and phosphorus content in tissues of juvenile grass carp(C. idellus)fed plant protein based diet.The results showed that，the feed conversion rate (FCR) was significantly lower, whereas protein efficiency ratio (PER) and specific growth rate (SGR) were significantly higher than those of other groups (P<0.05), when only monobasic calcium phosphate was supplemented. Compared with negative control group，after added phytase in diet, when the supplemented level of phytase was higher than 1 000 U/kg, the FCR of experimental group 3 (1 000 U/kg) was significantly lower than that of negative control group (P<0.05)，and PER was significantly higher than that of negative control group (P<0.05); The phytase has no effcets on the HSI, VSI, and CF of fish; the content of crude fat decreased in body, when the phytase content was higher than 1000U/kg, when the supplemented level of phytase content was 1 000 U/kg, the crude fat in muscle was significantly improved; The phosphorus content was enhanced to some degree in body, muscle and spine after added phytase, and when the content was 1 250 U/kg, the phosphorus content were significantly improved in muscle and spine. The result indicated that, when the supplemented level of phytase was 1 000 U/kg-1 250 U/kg, the SGR, PER and phosphorus content of grass carp were enhanced, and FCR was improved compared with negative control group. If you want to reduce the supplemented level of monobasic calcium phosphate, the suitable supplemented level of phytase was 1 000 U/kg-1 250 U/kg.