RHEED and its intensity oscillation are widely used for monitoring and controlling the growth by molecular beam epitaxy (MBE). We have discussed that the intensity oscillation is related to either the interference effect between two consecutive layers, surface step density or atomic density on a surface, depending on the diffraction conditions, i.e. the incident energy and the angle. We have also shown a result of multiple scattering calculation from large super-cells obtained by a MBE growth simulation, which should be important for more quantitative analysis of the intensity oscillation.