Based on numerical flow field and pollutant concentration field validations, three turbulence models (standard k-ε model, RNG k-ε model, standard k-ω model) were chosen to study Sc_t (turbulence Schmidt number) effects on the pollutant diffusion simulation in a street canyon using ANSYS Fluent software. The results show that the three turbulence models all perform well in simulating the flow and concentration fields. The concentration values change significantly with Sc_t and the optimum Sc_t value is 1.3 by which the best agreement can be got between the concentration field results and the wind tunnel experimental data. Because of the accurate prediction of the flow field with the standard k-ω model, the corresponding concentration results are also closer to the experiment data than those by the other two turbulence models. The optimum Sc_t value should be determined not only by the validation of concentration field but also by the validation of flow field.