The depth distribution of X-ray production φ(ρz) in some alloys was computed by means of Monte Carlo method and the modified expression of the total scattering cross-section and used for quantitative electron probe microanalysis. We have carried out computation on twelve kinds of binary alloys, one ternary alloy and carbon in the silicon carbide. The results given in the present work are preferable to both ZAF method and Shimizu's method. Especially, the computed results of carbon is much better than that computed by ZAF method. It follows that the present method can be used for quantitative microanalysis of light elements.