Microwave plasma depositions of diamond films have been investigated under low pressures of 10 mTorr to 10 Torr, at low substrate temperatures of 400 to 750°C, using high methane concentrations of 5 to 15% and oxygen concentrations of 5 to 10% in hydrogen plasmas. The deposition system consists of a microwave plasma chamber, a down-stream deposition chamber and a radio-frequency induction heated sample stage. The deposition system can be operated in either high-pressure microwave or electron cyclotron resonance (ECR) modes by varying the sample stage position. Cathodo-luminescence (CL) studies on diamond films deposited at 10 Torr pressure show that CL emissions at 430, 480, 510, 530, 560, 570 and 740 nm can be employed to characterize the quality of diamond films. High-quality, well-faceted diamond films have been deposited at 10 Torr and 600°C using 5% CH4 and 5% O2 in H2 plasmas; CL measurements on these films show very low nitrogen impurities and no detectable Si impurities. The diamond nucleation on SiC has been demonstrated by depositing well-faceted diamond crystallites on SiC coated Si substrates. The initial nucleation for diamond growth has been achieved by ECR plasmas under a low pressure of 13 mTorr.