Background & Objectives: MMR vaccine was added to Iranian children’s vaccination schedule from 2004 and there is no comprehensive research on adverse reactions of this vaccine in Iran. The aim of this study was determination of adverse reactions during one month after MMR vaccination. Methods: This is a cross sectional descriptive study that has been done on 210 MMR vaccinated children from January 2005 through December 2006 in Ardabil city. Samples were selected from both sex and two age groups 12-15- month and 4-6 years old. We used a checklist to collect demographic data as well as the parents contact numbers to follow possible adverse reactions 3-4 weeks after vaccination by phone call. Data were analyzed by descriptive statistical method using SPSS software. Results: 112(53%) children were boys and the remaining being girls. 34.76% of boys and 45.11% of girls were in the age group of 12-15- months and the remaining in 4-6 year of age. 92(43.8%) children developed adverse reactions. The most common reactions were: fever in 59 (28.09%), illness in 23(10.95%), injection site pain in 21(10%), rash in 14 (6.66%), nausea and vomiting in 10 (4.76%), and salivary gland and parotid swelling in 5 (2.38%) cases. Conclusion: In this study fever was the most common side effects reported after MMR vaccination and that caring of children for fever is recommended after vaccination.