Phytase could enhance the activity of digestive enzymes because it could hydrolyze the salts of phytic acid to release nutritional factors chelated by phytic acid, and eliminate the combination of phytic acid and endogenous digestive enzymes. The experiments were conducted to evaluate the effects of phytase on protease and amylase activities in Grass Carp (Ctenopharyngodon idellus) and NEW GIFT Nile Tilapia (Oreochromis niloticus) by feeding with plant-based diets supplemented with graded levels of phytase. Six isonitrogenous and isocaloric diets were formulated: two control diets (negative control diet: plant-based without calcium phosphate dibasic (DCP) and phytase added, and positive control diet with DCP added but without phytase), and the other four experimental diets with graded levels of phytase (250, 500, 1 000, 2 000 U/kg). Grass Carp of robust and uniform size with body weight 12.59±0.09 g and NEW GIFT Nile Tilapia with body weight 9.59±0.12 g were randomly divided into six groups respectively, 5 replicates per treatment with 20 fish. After 56 days breeding experiments, body weight of Grass Carp and NEW GIFT Nile Tilapia reached 18.29±0.63 g and 24.68±1.34 g, respectively. The stomach, hepatopancreas and intestine were randomly taken for analysis of endogenous protease and amylase activities. The results showed that in no-stomach fish like Grass Carp and gastric fish like Tilapia, endogenous protease and amylase activities were significantly enhanced by dietary phytase. By comparison, the sound effect of phytase in tilapia was more obvious, for the protease and amylase activities were improved remarkably by phytase at a low level of 250 U/kg for tilapia (P<0.05). Activities of these two digestive enzymes in Grass Carp and Tilapia tissues reached the maximum value at the level of 1 000 U/kg, while in Tilapia the enzyme activity values were not markedly different from those of the positive control (P>0.05).The protease activity in hepatopancreas of Grass Carp in 1 000 U/kg phytase group was significantly higher than that of positive control (P<0.05). In addition, the protease and amylase activities of Tilapia in 2 000 U/kg group was not significantly different from that of 1 000 U/kg group, while the protease activity of hepatopancreas in Grass Carp was significantly declined (P<0.05). Consequently, the optimum phytase amount in manufactured feed was 1 000 U/kg not only for NEW GIFT Nile Tilapia but also for Grass Carp under the condition of this experiment. The phytase could be used to replace partly inorganic phosphorus sources in the fish feeding.