We carried out this study to investigate differences of physiological variables between patients with (DM group) and without type II diabetes mellitus (Non-DM group) undergoing off-pump coronary artery bypass grafting (OPCABG). Postoperative $Mg^{++}$ and $Ca^{++}$ levels were lower, whereas $Na^+$ level was higher in DM group than those in Non-DM group. ICU (intensive care unit) stay time in DM group was longer than that of Non-DM group. Postoperative platelet counts tended to decrease, whereas C-reactive protein (CRP) and cardiac troponin-I (cTNI) levels tended to increase in DM group compared with Non-DM group. Postoperative albumin level was lower, while blood urea nitrogen (BUN) and creatinine levels were greater in DM group than those in Non-DM group. DM group had higher incidence of post-operative arrhythmias than Non-DM group. These results reveal that type II DM patients undergoing OPCAB may have higher incidences of postoperative hypomagnesemia, hypocalcemia and arrhythmias, and increases of CRP, cTNI, BUN, and creatinine levels than in Non-DM patients undergoing OPCAB. The perioperative check and control (supplement) of $Mg^{++}$ levels should be considered in cardiovascular surgery combined with DM.