Heavily carbon‐doped GaAs was obtained by low‐pressure metalorganic chemical vapor deposition by using trimethylgallium and arsine with a low V/III ratio. The hole concentration became as high as 2×1019cm−3without using intentional doping source gases when the V/III ratio was decreased to 2.4. The hole concentration coincided with the carbon concentration measured by secondary‐ion mass spectroscopy. The epilayer surface tended to be rough when the V/III ratio was decreased below 10. However, when the V/III ratio was lowered to 2.4, the epilayer surface became completely mirror‐like.