The nucleation and initial growth of diamond on Si(100) by biased hot filament chemical vapor deposition have been studied by scanning electron microscopy,Raman spectra,infrared absorption spectra and X-ray photoelectron spectra.After bias pretreated for 15min at -300V and 100mA,the nucleation density was found to be over 1010cm-2 on mirror-polished Si and was inhomogeneous,which can be divided into three regions on Si surface:A,cones at the edges;B,nanocrystalline diamond at the transition region between the edge and the centre;and C,SiC layers at the centre.The films with defects,textured films,and big discrete grains of diamond were formed at A,B,C regions respectively after 4h growth without bias.The nucleation enhancement by substrate negative bias is believed to be a combination effect of the ion bombardment and emission electron.The non-uniformity of ion distribution led to an uneven nucleation on the Si substrate surface.