One of the most common complications of operation and anesthesia is shivering. The purpose of this study was to compare the effectiveness of Ondanseton and Meperedine in preventing shivering after off-pump coronary artery bypass graft (OPCAB). In this double-blind randomized clinical trial, the sample consisted of 90 patients, who were candidates of CABG under general anesthesia. These patients were assigned to three groups, each containing 30 subjects: meperedine group (A), ondansetron group (B) and control group (C). Group (A) received 0.4 mg/Kg/IV of meperedine, group (B) received 8mg/IV of ondansetron and group (C) received Normal Saline. All these drugs were injected 15 minutes before the end of surgery. After the end of surgery, the intubated patients were transferred to the ICU and their body temperature was assessed through eardrum by a specialist who was blind to the research. The incidence of shivering in groups A, B, and C was 46.48%, 31.18%, and 60.83%, respectively (P=