The Reflection High Energy Electron Diffraction (RHEED) intensity oscillations under different angles of incidence and azimuths during Si(111) molecular beam epitaxy have been studied. The phase of intensity oscillation and initial transient response change dramatically with the angles of incidencl if the observation is carried out along 112 azimuthal direction, but vary little when observed along 011 azimuth. From the measurement results of RHEED specular beam rocking curve, we believe that, the characteristics of RHEED intensity oscillation as a function of electron diffraction condition, indicate the existence of two different seattering processes: coherently elastic diffraction beam oscillation and inelastic or diffuse scattering beam oscillation. This can be explained only by the electron multiple scattering mechanism. The origin of initial transient is also discussed.