This experiment was conducted to study the effects of adding phytase into diets with different levels of phytate phosphorus (PP) on the growth performance and serum biochemical indices of ducks. A total of 504 cherry valley ducks at the age of 1-day-old were randomly assigned to seven groups, which included one high non-phytate phosphorus (NPP, prior period, 4.0 g/kg; later period, 3.4 g/kg) control group and six low NPP (prior period, 2.4 g/kg; later period, 1.8 g/kg) experimental groups. Diets were formulated to contain two levels of PP (2.00 and 3.50 g/kg) and three levels of phytase (500, 1 000, 1 500 U/kg) by a 2×3 factorial arrangement. Each group had 6 replicates with 12 ducks per pen. The experiment lasted for 40 days. The results showed as follows: the PP and phytase levels significantly affected the average daily gain (ADG) (P<0.01) and average daily feed intake (ADFI) (P<0.01) in the whole period and feed/gain (F/G) (P<0.05) in the later period, and the high PP diet was superior to the low PP diet (P<0.05). There were significant interactions on final body weight (FBW), ADG, ADFI and F/G between PP and phytase levels (P<0.05). The ADG and ADFI in low PP diet were significantly improved (P<0.05) with the phytase levels increasing. The ADG and ADFI in 1 000 U/kg group of high PP diet were higher than those in 500 U/kg group (P<0.05), but the differences were not significant between the 1 000 and 1 500 U/kg groups (P>0.05). The phytase levels significantly affected the serum P level of ducks at 21 and 40 days of age and alkalescence (AKP) activity of ducks at 21 days of age (P<0.01) and did not significantly affect the serum Ca level of ducks at 21 and 40 days of age and serum AKP activity of ducks at 40 days of age (P>0.05). In conclusion, adding phytase in low NPP diets could significantly improve the growth performance of ducks, especially in high PP diet. With the phytase levels increasing, the serum AKP activity was decreased.［Chinese Journal of Animal Nutrition, 2010，22（4）:1063-1070］