Since metabolic pathway and enzymatic clarification of poly-phosphate accumulating organisms (PAOs) are still unclear, biological phosphorus removal (BPR) sometimes become unstable. We have focused on nitrite as one of unknown factors deteriorating BPR performance. And we obtained some findings from previous studies, namely 1) nitrite inhibits phosphate uptake and growth of PAOs, 2) nitrite inhibition persists even after nitrite disappearance, 3) PAOs with the higher relative anoxic activity are less sensitive to nitrite exposure. This study provides the modified Activated Sludge Model No.2d (ASM2d) to express nitrite inhibition properly by incorporating the assumed intracellular reaction product (reaction complex) of nitrite. The model also considers the tolerance mechanism of nitrite inhibition by the denitrifying activity of PAOs. From the results of previous experiments and relevant references, the modified model called “Nitrite-Complex model” incorporates new components, namely nitrite (SNO2) and reaction complex (Xcomplex), to ASM2d. The model incorporation only nitrite inhibition was able to fit with measured phosphate concentration while nitrite exists. Whereas, this model couldn't describe the persistence of nitrite inhibition after nitrite disappeared. However, result of nitrite-complex model incorporating nitrite and reaction complex inhibition, it fits well with measured phosphate concentration not only before nitrite disappeared but also after nitrite disappeared.