Background: Sever shivering is the most common adverse effect of spinal anesthesia during and after operation that causes physiologic complication. One of the drugs that could decrease operative shivering is ketamine. Objective: The aim of this study is the effect of ketamine to reduce shivering, during spinal anesthesia. Methods: This prospective randomized, and double-blind clinical trial was conducted on 355 patients of candida different surgeries with spinal anesthesia in Bu-Ali and Velayat Hospitals in Qazvin (2017). Patients were randomly divided into experimental (n=175) and control (n=180) groups. For the experimental and control groups, ketamine (0.2 ml/kg) or an equal volume of distilled water were administered with intravenous infusion pumps immediately after initiating spinal anesthesia, respectively. Then degree of shivering checked by anesthesiologist with visual scale during surgery. Findings: The rate and intensity of shivering at ketamine group significantly decreased compared to the control group. Shivering during surgery was less than in men, urological surgery and age (20-40 years) in both groups. Conclusion: Adding ketamine to drug's pump in patients undergoing spinal anesthesia could be effective method to control shivering during operation.