This study establishes a fashion theory frame to forecast future fashion cycles of pants through analyzing past fashion cycles of pants through a diachronic method. Pants pictures from 1967-2012, post-industrialized period of Korea were analyzed. Representative pant styles, fashion cycles of pants and the relation of pant styles, length and width were identified. The total of 1006 pictures in fashion magazine published over 46 years were selected and analyzed using PASW 18.0 (statistical program). The results are as follows. For 46 years, representative pant styles were skinny, regular and bell-bottom. The first cycle period was from 1969 to 1992 and lasted 24 years. The second cycle period was from 1993 to 2003 and lasted 11 years. The third cycle is ongoing as of 2004. Fashion cycles have shown a general trend to be shortened. The relation between pant styles, length and width revealed related results; in addition, pant length and width changed significantly in a similar orientation. Fashion marketers can develop successful products using fashion cycle theory from these results.