In our previous study (Lee et al., 2004), the changes in total P accumulation and P compounds with time in the plough layer in a paddy soil were investigated in relation to the continuous application of chemical fertilizers (NPK), straw based compost (Compost), combination these two (NPK + Compost). We found the contrast pattern of organic P accumulation between chemical fertilizer (NPK) and compost treatments (NPK+Compost, Compost). Organic P content increased significantly with time pass in chemical fertilizer treatment (NPK), but decreased with time in compost treatments, even though high amounts of organic P was applied repeatedly through rice straw compost. Microbial P contents and phosphatase activities in the fresh soils collected in April 2003 were comparatively higher in chemical fertilizer and compost combination, and compost treatment than in the chemical fertilizer. In this study, we tried to determine the reason why organic P accumulated more highly in chemical fertilizer treatment than in compost by using enzyme tests. Compost, and the combination of chemical fertilizer and compost accelerated the decrease in the organic P content in the surface soil, presumably due to promoting microbial activity. To investigate the effect of inorganic phosphorus applied by chemical fertilizer on phosphatase activities in the same treatments, we applied 0 and 20 ìmole K2HPO4 in fresh soils and determined phosphomonoesterase (PMEase) activity by using p-nitrophenyl phosphate(p-NPP). Acid PMEase activity was inhibited by max. 63 % in NPK by adding 20 ìmole K2HPO4 higher than in NPK+Compost (max. 46%) and Compost (max. 48%). We found from Michaelis-Menten equation that inorganic P (K2HPO4) inhibited competitively acid PMEase activities. As a result, the continuous application of chemical fertilizer might inhibit phosphatse activities in soil, which ultimately will increase organic P content in soil with time lapse.