Austenite-to-ferrite transformation in modern steels is a key metallurgical phenomenon as it can be exploited to produce microstructures that are closely associated with significant improvement of their properties. Both experimental and theoretical studies of this transformation have received much attention. In particular, in recent years, considerable efforts have been directed to the development of numerical models for adequate quantitative descriptions of the nucleation and growth of ferrite grains as well as the overall transformation kinetics. In this work, a modified multi-phase field model has been developed to simulate the isothermal γ-α transformation in a Fe-C alloy. This model takes both the effect of a finite interface mobility and a finite diffusivity into account, which hence enables a clear description of the mixed-mode nature of the transformation. In contrast to the diffusion-controlled phase transformation model, the carbon concentration in front of the moving γ-α interface is found to be non-equilibrium under this circumstance. In order to study the microstructural behavior and kinetics over the entire temperature range of the phase transformation, three different isothermal transformation processes have been imulated. The simulation results indicate that the nucleation density of ferrite increases with decreasing the temperature, which thus leads to a larger volume fraction of ferrite. However, the heterogeneous distribution of carbon in the untransformed austenite is intensified. The final microstructural product of the transformation at low temperature of 1010 K consists of fine residual austenite islands surrounded by fine polygonal ferrite. The simulation results also indicate that the transformation mode from austenite to ferrite varies from essentially diffusion-controlled at high temperature towards interface-controlled at low temperature.