An electrophotographic image forming apparatus including a photosensitive drum, a light generator to generate a light to be scanned on the photosensitive drum according to an image signal, an image signal supplier to supply the image signal to the light generator having a predetermined signal amount unit, a synchronous signal generator to generate a synchronous signal based on the light generated by the light generator, a light intensity signal generator to detect the light generated by the light generator and to generate a light intensity signal that corresponds to intensity of the detected light, a difference value calculator to compare the light intensity signal with a reference signal and to calculate a difference value of a signal level thereof, and a controller to control the image signal supplier to supply the image signal to the light generator if it is determined that a predetermined amount of time that corresponds to the difference value has elapsed since a time when the synchronous signal is generated. Thus, the electrophotographic image forming apparatus and an image forming method improves picture quality by compensating for variations in a supply start point of image signals that result from optical power changes.