A system for controlling an air/fuel ratio of an internal combustion engine supplied with a blended fuel of gasoline and alcohol and equipped with a blowby gas ventilator. An oxygen sensor is provided and an amount of fuel supply is corrected in response to the detected air/fuel ratio within a range having a lower limit set in the lean direction and an upper limit set in the rich direction. At engine starting, injected alcohol may not vaporize but leak into the crankcase and will then vaporize to pass into the air intake passage along with the blowby gas when the engine temperature rises, causing the air/fuel ratio to be become rich beyond the correction lower limit. For that reason, in this system, engine coolant temperature and alcohol content in the fuel are detected. And if the temperature is within a range defined from the alcohol boiling point when the alcohol content is above a reference value, the lower limit of the range is shifted in the leaner direction. The air/fuel ratio can therefore be corrected in the lean direction.