Diamond was synthesized on molybdenum substrate by chemical reaction of methane and hydrogen gas using the DC plasma jet at atmosphere pressure. The most important factors of diamond synthesis are the ratio of methane to hydrogen flow rates and the surface temperature of substrate. Experiments were carried out by changing the CH₄/H₂ ratios from 1% to 7% and diamond was well synthesized when the sluface temperature of substrate was 970℃. The substrate temperature was measured by optical pyrometer.