Introduction: There is controversy regarding the effects of insulin and metformin usage in postoperative hyperglycemia after coronary artery bypass graft surgery. The purpose of this study is to compare the effect of insulin and metformin on blood acidity, bicarbonate, potassium and magnesium in nondiabetic patients undergoing coronary artery bypass graft surgery. Materials and Methods: In a clinical trial, 56 nondiabetic patients who underwent coronary artery bypass graft surgery in the Kowsar Hospital in Semnan and suffered post-operative hyperglycemia, randomly received insulin and metformin. Levels of blood glucose, acidity, bicarbonate, potassium and magnesium were measured and recorded for both groups before and after surgery. Data was analyzed (significant level 0.05) using Chi-square, Mann-Whitney and Friedman tests. Results: At the time of admission of patients in the ICU, means of arterial blood acidity, bicarbonate, potassium and magnesium were 7.37±0.07, 21.8±2.5 mEq/l, 4.3±0.5 mmol/l, and ± 2.1±0.3 mg/dl respectively in the insulin group and in the metformin group these values were 7.40±0.05, 22.2±2.8 mEq/l, 4.1±0.5 mmol/l, and 2.1±0.4 mg/dl respectively. There was no significant difference between the means of potassium, magnesium and bicarbonate in the two groups at different times (P>0.05). However, there was a significant difference between the mean values of arterial blood acidity 72 hours after admission in ICU in the two groups of insulin (7.41±0.03) and metformin (7.39±0.04) (P=0.047). However mean PH of the blood remained normal. Conclusion: In nondiabetic patients who have hyperglycemia following coronary artery bypass graft surgery, the use of metformin and insulin does not affect bicarbonate, potassium and magnesium of blood. However, overtime the use of metformin may gradually increases the pH of the blood. Therefore, it is recommended that metformin should be used in patients with blood pH control.