In this paper, we report the in-situ study of the dependence of electrical resistivity of magnetron sputtering Mo film on the thickness. We measured the electrical resistivity of Mo film by vacuum measurement. After theoretical computer fitting, we obtained the t heoretical curve of the dependence of resistivity of Mo film on the thickness. We compared the theoretical curve with the experimental curve, the results show that when the thickness is large, there is a very small diversity between the theory of Fuchs-Sordbeimer of resistivity and the present thickness dependence of electrical resistivity. If the effect of grain boundary is considered, the result will agree with the Fuchs-Sordbeimer theory very well in small thickness. We also found that the process of conductance is the thermal electron emission, when the film is not a successive film.