The growth dynamical processes of Si(111) molecular beam epitaxy under different growth temperature are studied by RHEED intensity oscillations. The Si(lll) epilayer grew in a "step flow" mode at the temperature above 520℃(growth rate 0.02 nm/s), and in a "2D nucleation" bilayer mode at temperature range between 520℃ and 420℃. Below 380℃, even at room temperature, the growth is still in a bilayer mode. The RHEED intensity oscillation shows a monoatomic layer mode during the initial stage of growth, which is believed due to the superposition of two kinds of oscillations originated from coherent elastically scattering and incoherent scattetring or surface diffuse scattering.