An assumed time scale model is applied in the numerical calculation of the turbulenceround jet flow, the results are found to be reasonable. Then, joint-scalar probability densityfunction(pdf) approach together with the assumed time scale model was employed to computeturbulence diffusion flames, several mixing models were compared. It was suggested thatscalar time scale should be used to improve mixing models. An assumed time scale model is applied in the numerical calculation of the turbulenceround jet flow. The results are found to be reasonable. Then, joint-scalar probability density function (pdf) approach together with the assumed time scale model was employed to compute damage diffraction flames, several mixing models were compared. It was suggested thatscalar time scale should be used to improve mixing models.