In the widely used Activated Sludge Models (ASMs) for biological wastewater treatment, denitrification is assumed as a single-step process, which is not true for some cases where a considerable amount of nitrite is build-up in system. This causes limitation to the application of ASMs. In this work, with a consideration of denitrification on both nitrite and nitrate, and an introduction of the simultaneous storage and growth concept, a two-step ASM3-based denitrification model is established. The sensitivity of the effluent chemical oxygen demand, NO2- - N and NO3- - N concentrations toward the stoichiometric and kinetic coefficients is analyzed. Model calibration is performed by comparing the measured and predicted values for model components. Thereafter, this model is evaluated with the experimental results of four different case studies on denitrification and compared with the original ASM3. The evaluation and comparison results demonstrate that the developed model is able to better and more mechanistically describe the denitrification process in the activated sludge systems.