A de-skew method of an image forming apparatus is provided which includes reading an image on a print medium fed through an automatic document feeder (ADF); detecting a leading edge of the print medium and calculating a skew angle of the read image; comparing the calculated skew angle to a first threshold and determining whether to perform a de-skew operation; and setting skew compensation levels based on the calculated skew angle and performing different de-skew operations according to the set skew compensation levels to output a de-skewed image when the absolute value of the calculated skew angle is equal to or greater than the first threshold.