Abstract: Low temperature deposition of diamond thin films in the range of 700-300℃ has been successfully carried out by microwave plasma assisted CVD technique. It was found that the introduction of oxygen into the system of CH₄-H₂ not only lowered or even eliminated the diffused peak at 1550cm^-1 caused by non-diamond phases, but also reduced the half height width of the diamond peak at 1332cm^-1. It was because that the atomic oxygen did have a much stronger etching effect on graphite and diamond-like carbon than that of atomic hydrogen at relatively low temperatures, could the low temperature growth of diamond from the vapour phase become possible. Low temperature deposition and characterization of the resultant diamond films are reported in detail. An attempt to link the low temperature deposition data to the C-H-O phase diagram recently proposed by Bachmann et al. has also been made.