In a method of determining a focus position for a substrate exposure process and a substrate exposure apparatus capable of performing the same, a reticle having a light-transmitting region may be illuminated by an off-axis illumination light. A projected light, which is transmitted through the reticle and a projection optical system, may be detected by a light sensor disposed on a substrate stage. An intensity of the projected light measured at a light-receiving surface of the light sensor may vary in accordance with positions of the light-receiving surface. The focus position may be determined based on the variations in the intensity of the projected light.