Abstract: An experiment was conducted to determine effects of non-phytate phosphorus (NPP) and microbial phytase on performance, nutrient utilization, tibia development and plasma parameters in 1 to 21-day-old broiler chicks fed with a corn-soybean meal diet and to evaluate the equivalency values of microbial phytase for inorganic phosphate (Pi). On the day of hatch, 400, mixed sex, Arbor Acres chicks were allotted into treatments of ten chicks per pen with four pens per treatment. Ten experimental diets were formulated. Diet 1 (basal diet) to 4 contained 2.1, 2.9, 3.7, and 4.5 g/kg NPP. The PP content in all diets was 2.4 g/kg. To the basal diet, diet 5 to 10 was supplemented with microbial phytase 250, 500, 1 000, 2 000, 4 000 and 8 000 U/kg, respectively. The results showed that body weight gain, feed intake, feed conversion rate, plasma Pi concentration, tibia length and strength, tibia ash weight and content, tibia P, and total P retention increased, but mortality and plasma alkaline phosphatase activity decreased with dietary NPP or phytase levels increasing. The relationship between dietary NPP or phytase levels and broilers growth was quadratic. When 8 000 U/kg phytase added, about 94.84% phytate phosphorus was hydrolyzed, and broilers growth performance was equal to those fed with diet of 4.5 g/kg NPP. The model to predict inorganic phosphate releasing from phytate phosphorus by microbial phytase was as follows: y = 0.425 4 log₂(1 + x/250) + 0.283 （n=7, R2 = 0.915 3，P=0.000 7），where y is the phosphate releasing (g/kg) and x is the microbial phytase level (U/kg). This experiment indicated that high level microbial phytase could hydrolyze almost all phytate phosphorus and the model could be helpful to poultry diets formulation for nutritionist and managers.