Postoperative atrial fibrillation (POAF) is the most common complication after cardiac surgery, despite improvements in anesthesia, surgical techniques, and medical therapies. Although beta-blockers have been proven to be effective, the incidence of POAF is around 20 % even with these agents. The mechanism of POAF is not fully elucidated and no optimal strategy has been established for POAF. There are two important elements of "structural" and "electrical" remodelling of the atrium in the mechanism of POAF. A patient's age and preoperative left atrial fibrosis can predict POAF associated with structural remodelling. Although inflammation and oxidative stress during cardiac surgery may be the underlying mechanisms for electrical remodelling causing POAF, there are no reliable clinical parameters for their detection. Nonetheless, postoperative P-wave dispersion and electromechanical delay, which reflects excitation-contraction coupling abnormalities, could be new parameters for POAF. In conclusion, despite the importance of prevention of POAF, there are only a few parameters for predicting POAF. It is therefore necessary to consider both disease-mediated structural remodeling before surgery and electrical remodeling caused by cardiac surgery.