Diamond thin films were deposited on silicon wafers by microwave plasma enhanced chemical vapor deposition (MPECVD). The silicon wafers were pretreated by ultrasonic vibration using diamond powder. The diamond thin films were deposited at 40 Torr, 1100 W microwave power using dilute gas mixtures of methane and oxygen in hydrogen. Methane concentrations varied from 0.2% to 5% in hydrogen and oxygen concentrations of 0%–5%. These diamond films were characterized by Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The majority of the diamond crystals had many defects, including {111} twins and stacking faults. The density of these defects increased with increasing methane concentration during the deposition process. Multiply twinned particles were observed with five growth sectors presenting fivefold symmetry. In interfacial studies, the defects in the diamond thin films were seen to fan out from a small region at the interface, implying a nucleation site.