We deposited diamond phase carbon thin films on Si substrate by the electrolysis of methanol solution. A little amount of ammonia solution was added to increase the current density of the electrolyte. We analyzed films by XRD and SEM. The chemical change of electrolyte during the electrolysis process was characterized by FTIR. We obtained better quality diamond phase carbon films at a lower applying boltage(300V) and temperature ($40^{circ}C$) by adding ammonia solution to methanol electrolyte. Diamond (111), (220), (311) peaks were shown distinctively in XRD graph. Addition of ammonia solution resulted in lowering the applying bias voltage to 300V and the substrate temperature to $40^{circ}C$ still maintaining a high current density at 80mA/$ extrm{cm}^2$, which prohibited a great loss of solution from vaporization. Possible change of chemical reaction due to the addition of ammonia solution was also discussed.