This study was executed to research the evaluation and additivity of available phosphorus (P) in feedstuffs of plant origin for ducks with emptying-force-feeding method. Eight-four 21-day-old ducks with similar weight and excreta-collecting apparatus were randomly assigned into 7 treatments with 6 replicates in each treatment and 2 ducks in each replicate. The ducks under trial were starved for 72 h. After fasting period, the ducks from different treatments were fed by gavage with 70 g corn, soybean meal, cottonseed meal, rapeseed meal or their equal proportion mixtures, which included corn-soybean, corn-soybean-cottonseed, and corn-soybean-cottonseed. Excreta was collected separately for 48 h and bioavailability of P was evaluated. The results showed that for mixed diets corn-soybean-cottonseed, there was significant difference between analyzed and calculated values for the apparent phosphorus availability, true phosphorus availability, apparent available phosphorus and true available phosphorus (P<0.05), while there was no significant difference between the other two mixed diets (P>0.05). It was concluded that, 1) the additivity of true available phosphorus was better than that of apparent phosphorus for different feedstuffs of plant origin; 2) the additivity of apparent available phosphorus and true available phosphorus were variable for different combination of feedstuffs. ［Chinese Journal of Animal Nutrition, 2010，22（4）:917-922］