Objective Analyze the causes of the conversion from off-pump coronary artery bypass (OPCAB) to on-pump coronary artery bypass grafting (ONCAB), in order to improve the operative survival rate. Methods From January 2000 to December 2003, 1952 patients underwent OPCAB. 29 patients (1.49%) required conversion from OPCAB to ONCAB. The causes of conversion included ventricular fibrillation in 5 cases, and hypotension in 24 cases. 26 patients received CABG under CPB. 8 patients under aortic cross clamping, and 3 patients needed only CPB support. Results The mean number of grafts was 2.9?0.6. 9 patients died, postoperatively with a mortality rate of 31.03%. In patients with the perioperative blood pressure lower than 80/40?mm?Hg, the mortality rate was 47.6%, and in patients with blood pressure higher than 80/40?mm?Hg, the mortality rate was 8.33% (P=0.043). No one with duration of hypotension less than 15min died, the mortality of the patients with hypotension times more than 15min was 42.86% (P=0.029). In the death group, all the patients had more than three vessel disease and severe vessel stenosis or occlusion. The mortality was hither in female patients. Conclusion Patients converting from OPCAB to ONCAB had a higher mortality. The severity and lasting time of hypotension is closely correlated with the mortality rate. The high risk factors included the surgeons with less OPCAB experience, the patients with more than three vessel diseases and severe coronary stenosis or occlusion, and female patients. The indication of OPCAB must be strictly controlled. Hypotension and severe ventricular fibrillation occurred during OPCAB must be corrected and eliminated immediately.