A head adjustment method of adjusting overlaps of a plurality of print heads, which are arranged in a widthwise direction of a print medium in an image forming apparatus, in the widthwise direction. The head adjustment method includes setting a theoretical input pattern image to be formed by the print heads, outputting a practical output pattern image on the print medium by driving the print heads according to the input pattern image, estimating practical overlap values of the print heads through the output pattern image, and adjusting degrees of overlap between the print heads according to the estimated overlap values.