BACKGROUND: Propofol is used as an induction and maintenance agent for general anesthesia but it can cause adverse reactions like hyperlipidemia, growth of microorganisms, and pulmonary embolisms. Microemulsion propofol was developed to avoid these side effects but incidence and severity of pain on injection is higher than with lipid emulsion propofol. We aimed to compare the effects of sufentanil in analgesic doses for reducing the injection pain of microemulsion propofol. METHODS: The candidates included eighty patients, 19-60 years old and ASA I-II. They were randomly classified into four groups and pretreated with normal saline, sufentanil 0.1 microg/kg, 0.2 microg/kg or 0.3 microg/kg before injection of microemulsion propofol. Five minutes after receiving pretreatment drug, 2 mg/kg of microemulsion propofol was injected and VAS was recorded. RESULTS: There were no significant differences in the incidence of injection pain among the groups. Severity of injection pain was significantly lower in the sufentanil 0.3 microg/kg group than normal saline and sufentanil 0.1 microg/kg group. Significant differences in blood pressure and heart rate were observed in sufentanil groups only after endotracheal intubation. One patient each in sufentanil 0.1 microg/kg and 0.3 microg/kg group experienced mild cough, one from sufentanil 0.3 microg/kg group experienced dizziness and another showed signs of hypoxia. One patient each in normal saline and sufentanil 0.1 microg/kg group showed clinical symptoms of phlebitis in the injection area. CONCLUSIONS: Pretreatment with sufentanil 0.3 microg/kg reduced the severity of microemulsion propofol injection pain without increasing arterial blood pressure and heart rate after endotracheal intubation.