Diamond films were synthesized by microwave plasma chemical vapor deposition (CVD) on carbon steels with various surface treatments, including gas carbonitriding, carburizing and TiN coating. Films were analyzed for their crystallinity using Raman microprobe spectroscopy. On pure iron substrates and carbon steel with gas carbonitriding and carburizing, diamond films containing amorphous carbon grew over the powder-like graphite that was formed. On TiN-coated carbon steel substrates, diamond films with excellent crystallinity grew without the formation of the powder-like graphite. This suggests that the TiN layer inhibits Fe atom diffusion. Crystallinity and the ratio of amorphous carbon contents of diamond film have little dependence on the carbon content of the steels used in this study.