The critical times of phosphorus segregation in low alloy steel 12Cr1MoV at different solution temperatures, 1050°C and 1300°C, are studied and the diffusion coefficient of complexes, Dc, and diffusion coefficient of phosphorus, Di, are calculated. Results show that the calculated result with the kinetic equations perfectly fits with the experimental observations. And that the higher the solution temperature, the higher is the segregation level of phosphorus, and also the longer is the critical time. It also shows that in the segregation process, the rate of phosphorus segregation at first is high due to a large diffusivity of the complex, Dc. Then the segregation rate decreases due to decreasing complex concentration gradient. In the de-segregation process, the level of phosphorus segregation decreases with increasing holding time t. But the rate of phosphorus desegregation from grain boundaries to centre is slower compared with that in segregation for the diffusion coefficient of phosphorus atoms, Di, is far lower than that of the complexes, Dc.