Background: Postoperative atrial fibrillation (POAF) is the most important type of secondary AF. POAF is a complication in approximately one-third of patients undergoing cardiac surgery. The duration from P-wave onset on ECG to the peak of the A' lateral wave on tissue Doppler imaging (PA-TDI) provides a reliable estimation of total atrial conduction time. Objective: To evaluate the efficacy of the PA-TDI duration for predicting post-operative atrial fibrillation in patients undergoing cardiac surgery. Patients and methods: This prospective study was conducted at national heart institute for 70 patients who were eligible and planned for undergoing cardiac surgery. Patients were divided into 2 groups according to the presence of post-operative AF. Accordingly, patients in the study were divided into group (I) that included patients who developed POAF and group (II), which included patients who remained in sinus rhythm and did not develop POAF. Results: PA-TDI duration in group I was longer than in group II with highly significant difference between the two groups with P-value < 0.001. It was found that PA-TDI duration (≥ 171.5 ms) has sensitivity equal to 91 and specificity 95%. Conclusion: PA interval measured by TDI preoperatively seems to be a simple method to predict incidence of POAF in patients undergoing cardiac surgery and it may be part of preoperative cardiovascular examination. The current study suggests that longer PA-TDI duration to be additive to conventional risk factors and biomarkers in predicting POAF and that PA-TDI duration is a predictor of POAF, and the best cut off value of PA-TDI duration for POAF is 171.5 ms.