Diamond nucleation on untreated silicon was enhanced by positive de biasing in a horizontal microwave plasma chemical vapor deposition system. The effects of process parameters of bias voltage, methane content, bias time, working pressure and microwave power on the diamond nucleation were investigated. A nucleation density higher than 10(10) cm(-2) was achieved on an untreated, mirror-polished silicon substrate by positive de biasing. With a bias voltage between the range of + 200 and + 300 V,it was found that high methane content is effective in the enhancement of diamond nucleation. It was also found that amorphous carbon was formed under the positive de bias conditions. Uniform diamond films were obtained by combining the positive de bias enhanced nucleation process with subsequent growth under the usual microwave plasma chemical vapor deposition diamond conditions. Diamond films deposited were characterized by scanning electron microscopy and Raman spectroscopy. The effect of positive de biasing on nucleation at a high methane content to generate oriented diamond nuclei on (100) Si substrate was explored.