Background and Objective: Newborns often experience pain during routine invasive procedures. Although the pain caused by immunization injections could be controlled, but this procedure in infants is often done without pain relief. The aim of this study was to assess the effect of 20%oral sucrose on the relief of pain from Hepatitis Bvaccine injection in full term infants. Material and Methods: In this randomized clinical trial, 60 infant were selected and divided randomly into two groups. In the case group, 2 ml of 20% sucrose were given orally to the infants 2 minutes prior to the intramuscular injection of Hepatitis B vaccine. By contrast, the control group received 2 ml of sterile distilled water instead of sucrose. Infants’ pain levels were assessed with the Neonatal Infant Pain Scale (NIPS) prior to the administration of the sucrose or water, as well as immediately after and 5 minute after the injection. Results: Our findings show that there were no significant differences in the pain levels prior to the injection between the two groups regarding sex, gestational age, birth weight, mode of delivery, and postnatal age. There were also no significant differences in pain severity immediately after the injection between the two groups. However, 5 minute after the injection the average level of pain severity in the 20% sucrose group was significantly lower than the control group (P = 0.012). Conclusion: Oral administration of 20% sucrose does not have an effect on the pain level immediately after intramuscular injection, but it relieves the pain 5 minute after the injection.