Diamond particles 10–500 nm in diameter were produced by microwave‐assisted combustion of acetylene in oxygen. Both premixed and diffusion flame configurations were investigated. A mixture of cubic and hexagonal polytypes of diamond were identified. Larger particle sizes were observed at lower reactor pressure and higher C to O atomic ratios. C to O atomic ratios between 0.83 and 1.0 produced crystalline diamond powder while other ratios produced graphite, soot, and amorphous carbon phases. Diamond formation was not observed when reaction pressures were above 150 Torr.