An apparatus and a method for determining an overlap distance of an optical head is disclosed. Positions and light amount distributions of each light spot can be measured, which may be provided from an optical head to a substrate. Gaussian distribution may be applied to the positions and the light amount distributions to calculate a compensation model of each of the light spots. A first accumulated light amount corresponding to each first area of the substrate may be calculated if the optical head is scanning along a first direction of the substrate using the compensation model. A second accumulated light amount corresponding to each second area overlapped with the each first area is calculated if the optical head is scanning along the first direction, which is moved in a second direction by a first distance using the compensation model. An overlap distance may be determined based on a uniformity of summations of the first and second accumulated light amount.