This study proposes a methodology to estimate time-varying in situ wear coefficient between steam generator tubes in nuclear power plants and their supporting structures. Actual wear depth measurement data of steam generator tubes of OPR1000 (Optimized Power Reactor 1000 MW) plants in Korea were collected and analyzed to investigate the behavior of fretting wear. To determine the in situ wear coefficient, a mathematical expression was developed as a function of various parameters such as measured wear depth time history, work rate, contact geometry of the tube, and its support. These calculated in situ wear coefficients were then used to obtain wear depth history curves. Results obtained were then compared with actual field measurement data to show the validity of the proposed method. Many researchers have obtained wear coefficients under laboratory conditions. However, those coefficients cannot be considered as realistic factors for operating steam generators. The in situ wear coefficient proposed in this study is based on wear measurement data obtained from real operating steam generator tubes. Therefore, they can be used to precisely predict the wear depth of steam generator tubes, thus allowing safe and economical management of steam generators.