Abstract: The diffusion of passive scalar in shearless turbulent mixing layer is computed by DNS (direct numerical simulation). The effect of Schmidt number on the turbulent diffusion of passive scalar is studied together with experimental data in wind tunnel with incense as passive scalar. Sc number equals 0.2 and 3 respectively in DNS. Comparing the results of different Sc numbers including the data of high Sc number in experiment, it is found that the mean scalar profile is almost independent of Sc number, and the variance of fluctuated scalar concentration increases with Sc number. It is also observed that non-dimensional mass flux is almost independent of Sc number when Sc number is greater than 1, while the non-dimensional mass flux of Sc=0.2 is different from that of Sc>1, because the molecular diffusion effect takes place when Sc number is very small.