The effects of surface pretreatments on the nucleation of diamond on silicon substrates have been studied by quantitative X-ray photoelectron spectroscopy (XPS), SEM and Raman spectroscopy. The methods of surface pretreatments including ultrasonic abrasion, scratching, and DC biasing were used for diamond nucleation enhancement. Diamond films grown by different surface pretreatments and selected intervals in bias-enhanced nucleation were analyzed for the surface composition in the C 1s and Si 2p regions. Before diamond growth XPS analysis showed that the Si substrate surface is covered by a layer of SiO2 and carbonaceous residue. It was found that methods of surface pretreatment all introduce a substantial amount of carbon species on the substrate surface that was the primary reason for the enhancement of diamond nucleation. In the biasing process, it reduced and suppressed the formation of oxide that further contributed to the enhanced nucleation density of diamond, and it was observed that the presence of SiC in the initial stage of nucleation is due to the carbon interaction with the Si substrate.