8 red soils differing in fertility levels were tested to study the relationships between soil microbial biomass P and soil P or P plant availability. The results showed that red soil microbial biomass P was closely related with soil total P, soil organic P and soil rapidly available P. Relative coefficients were 0.840, 0.897 and 0.944, respectively. Especially, the significant relation of red soil microbial biomass P to soil rapidly available P suggested that red soil microbial biomass P may act as an indicator of P-supply ability. Greenhouse incubation expriments showed that red soil microbial biomass P was also closely related with ryegrass dry matter yield, ryegrass P uptake and P uptake per unit weight of ryegrass. Relative coefficients were 0.789, 0.895 and 0.910, respectively, which indicated that P plant availability of red soil microbial biomass P was not only reflected on ryegrass dry matter and plant P uptake, but also more obviously reflected on P uptake per unit weight of plant.