The diamond nucleation and growth on Si substrate by hot filament chemical vapor deposition (HFCVD) at the low temperature ( ～ 550℃ ) and low pressure ( ～ 7 Torr) were studied. Nucleation density (ND) as high as 1.5 × 1011cm-2 was obtained on well ultrasonically pretreated substrate at the nucleation conditions of 2.5% CH4/H2. Diamond grain sizes change form sub-micron to nano-meter scales with the increase of CH4 concentration. Smooth ultra-thin (thickness ＜500 nm) nano-crystalline diamond (NCD) films with grain sizes less than 50 nm and surface roughness as low as 4nm have been synthesized. The adhesion between the film and substrate is good. The optical absorption coefficient in visible (VIS)to infrared (IR) wavelength range is less than 2 × 104cm-1. Smooth ultra-thin NCD films can be synthesized at low temperature and low pressure by our conventional HFCVD technique.