The requirements and optimum ratio of calcium to phosphorus in the compound feed of grass carp ( Ctenopharyngodon idellus) were studied. Results were analysed based on the two biological indexes of weight gain,feedconversionratioandfeedefficiency. Resultsshowthecalciumrequirementis 0.473 %-0.788 %, Phosphorus requirement is 1. 419 % -1. 577 % , total requirement is 2. 1 % -2. 7 %. The optimum ratio of calcium to phosphorus changed with the levels in feed. When calcium and phosphorus levels were lower , but total level was different , the optimum ratio of calcium to phosphorus is 1∶3. When calcium and phosphorus levels were higher , but total level was same , the optimum ratio of calcium to phosphorus is 1∶2. When the phosphorus value is below 2 in the ratio,higher total level of calcium and phosphorus can decrease the stress of grass carp because of imbalance of calcium and phosphorus. When the ratio of calcium to phosphorus is not 1∶2, or 1∶3, higher or lower total levels of calcium and phosphorus can affect the growth and feed conversion ratio of grass carp .