Abstract: Phosphate application in environment enhancing nodulation and nitrogen fixation of leguminous plant Sebania Cannabina (Retz.) pers. was not unlimited. It was inhibited by the available-P level in the environment.Related with the 'PAENFAL' effect of Sesbania Cannabina (Retz. ) Pers. , the critical available-p value (O.1 N HCL method) of lateritic red soil in plowed layer in the granitic area of Guangzhou was 25.6 ppm P. Also in nitrogen-free nutritional solution, ( 1 ) the critical phhosphorus concentration (mM P) were 0.78 and 0.48, and (2) the critical phosphorus content of plant (P%) were 0.213 and 0.157 separately in Autumn and Spring. These two critical values trended to be influenced by the seasonal chanbe The. significant decrease of fixed nitrongen by this plant was observed in the excessive-P solution. Significant promotion effect of increasing phosphorus supply on the nodulation, total dry weight of nodules and nitrogenase activity were obtained in low-P solution. This promotion effect was then leveled within a range of phosphorus supply. No harmful effect on nodulation was observed in the excessive-P supply but which inhibiting effect was appeared, not only on the total dry weight of nodules, but also on the nodule nitrogenase activity more seriously.