Atomic steps of ZnSe surface growing on vicinal GaAs(100) substrate are investigated with in-situ reflection high-energy electron beam diffraction. The diffraction patterns indicating 1 ML-height step with uniform terrace width are clearly observed as the growth proceeds. The uniformity of step array on growing surface becomes stable by ZnSe growth of 500 Angstrom. The terrace width and substrate vicinal angle measured by the diffraction patterns agree well with the substrate nominal values. The observed diffraction patterns are compared with simulation results based on kinematical diffraction theory to discuss the fluctuation of surface step structure. The improvement of uniformity of step array is anisotropic with regard to the substrate vicinal direction. The insertion of CdSe layer does not cause broadening of full width at half maximum of reciprocal lattice rod, which indicates that the uniformity of step array is unchanged.