In this paper, a new method for determining the crystallite size from line profile analysis is presented. The diffraction line profiles are fitted by weighted Gauss-Cau-chy combination functions. By using Fourier transform method, the integral width of pure physical brodening function f(x) can be obtained directly without assumption about the profile f(x). The method is more accurate, simple and convenient than the usually used methods of approximate functions, and the artificial and arbitrary nature of the assumption about function f(x) are avoided.