Abstract: The trial was conducted to study the effects of phytase on growth performance and determine the phosphorus equivalent of phytase in channel catfish (initial average weight about 1.70 g). Single factor design was used for the trial, Ca(H2PO4)2 was provided in four inorganic phosphorus diets (0.3%, 0.5%, 0.8% and 1.2%), and different levels of phytase were sprayed on basal diet (300, 500, 1000, 1500 and 2000 U/kg). To determine the optimum additive amount of phytase by line model, fish of homogeneous sizes were randomly allotted to 10 dietary treatments with triplicates of 30 fish in each tank. The linear relationships were established between supplemented P and response parameters (specific growth rate and vertebra bone phosphorus) to evaluate phytase phosphorus equivalent. The results showed that: (1) Compared with the basal group, growth rate, specific growth rate, protein efficiency ratio and condition fatness were increased significantly (PPPY1=135.94X+7.8316 (X-inorganic p, Y1-SGR, R2=0.9453), the phosphorus equivalency Ca(H2PO4)2 of 300, 500, 1000, 1500 and 2000 U/kg diet phytase was 0.13%, 0.57%, 0.76%, 1.46% and 1.35% respectively, and equal to 0.03%, 0.14%, 0.19%, 0.36% and 0.33% available phosphorus added to basal diet; Y2=0.8737X+5.1028 (X-inorganic p, Y2- vertebra phosphorus, R2=0.9638), the phosphorus equivalency Ca(H2PO4)2 of 300, 500, 1000, 1500 and 2000 U/kg diet phytase was 0.47%, 1.11%, 1.18%, 1.38% and 1.41% respectively, and equal to 0.12%, 0.27%, 0.29%, 0.34% and 0.35% available phosphorus added to basal diet. In conclusion, 1000 U/kg-2000 U/kg phytase supplemented in diet significantly improved growth performance, conduced to the deposition of nutriment in body and bone mineralization. The phosphorus equivalency of 1435 U/kg diet phytase was the optimum and equal to 0.37% available phosphorus added to basal diet if mean specific growth rate was as reference index, then the phosphorus equivalency of 1226 U/kg diet phytase was the optimum and equal to 0.33% available phosphorus added to basal diet if mean specific growth rate was as reference index.