An investigation on the dynamics of transformation of P from ³²P-labelled ryegrass in red soils was conducted in laboratory. The results showed that the rapid increase in flush ³²P related with biomass P was accompanied with the decrease in extractable ³²P on the first 3d of incubation in both sandy and clayey soils, and afterwards, it displayed great fluctuation in sandy soil, but had little fluctuation in clayey soil during 3〜20d of incubation. At the later stage of incubation, the increase in extractable ³²P was accompanied with decrease in flush ³²P. The opposite changes in content of extractable ³²P and flush ³²P suggested transformation of ryegrass P was closely related to its utilization and its release from microorganisms in red soils. It can be concluded that addition of organic matter accelerated the release of soil native P according to the changes in the extractable soil P during incubation.