The nucleation and growth of diamond on two-step pretreated Si substrates were studied. The diamond films were produced by hot-filament chemical vapor deposition. The two-step pretreatment comprised coating the smooth Si substrate with a thin carbon film by electrolysis of methanol solution, followed by 1 min ultrasonic treatment with diamond powder. An enhanced diamond nucleation density as high as 2 X 10(9) cm(-2) was obtained, three orders of magnitude higher than that on single-step pretreated Si substrates. In addition, rapid diamond nucleation on the pretreated Si substrate was observed. Explanations are given for the experimental results and a scheme of the kinetic process of nucleation is proposed.