In this paper we present a new model for interpreting Lau effect on the basis of partial coherent transfer function in free-space propagation. The generalized formula for description of intensity distribution of Lau fringe is deduced. It is suitable for arbitrary periodical amplitude or phase objects. The condition of producing Lau fringe and output intensity for some typical periodical objects is analysed. The experimental results of Lau fringe produced by line gratings with different periods are given.