Purpose: The purpose of this study was to examine the effect of a blood glucose control protocol for medical intensive care unit (ICU) patients. Methods: The subjects were recruited from medical ICU adult patients whose blood glucose levels exceeded 200 mg/dL in two consecutive tests. The experimental group (n=62) received the modified Yale (MY) insulin protocol, whereas the control group (n=64) was treated with the conventional insulin therapy methods. Results: In the experimental group, the mean blood glucose levels (p<.001) and the time to reach the target range of glucose (p<.001) decreased significantly while the incidence rates of a target range of glucose of 100-140 mg/dL (p<.001) increased significantly as compared to the control group. However, no statistically significant differences were found in the incidence of hypoglycemia(p=.644), or the number of glucose tests (p=.236) between the groups. The length of stays in the ICU (p=.001), ventilator care days (p=.038), and the Sequential Organ Failure Assessment (SOFA) score (p=.029) in the experimental group were significantly lower than those of the control group. Conclusion: Application of the protocol was effective in improving the state of blood glucose control in medical ICU patients. Therefore, this protocol is expected to be used as a part of nursing intervention in critical care nursing.