Abstract: Objective To monitor the antibody levels against measles, mumps and rubella before and after the booster immunization of measles-mumps-rubella (MMR) vaccine in children aged 6 years in Changping district of Beijing, and provide evidence for the control of related diseases and the improvement of immunization strategy. Methods Healthy children aged 6 years were selected in a multistage stratified sampling in 6 townships in Changping. A total of 308 children were selected for pre-vaccination antibody level detection by using venous blood samples (3 ml for each) collected from them based on the principal of informed consent, and the IgG levels against measles, mumps and rubella were detected by using ELISA kit. After booster immunization, 274 venous blood samples (3 ml for each) were collected from them at 35th-42nd day to detect the IgG levels. Results Before the booster immunization, the positive rates of measles, mumps and rubella antibodies were 94.16%, 87.99% and 72.40%, while the geometric mean concentrations(GMCs)were 999 mIU/ml, 469 U/ml and 44 IU/ml, respectively. After the booster immunization, the positive rates of measles, mumps and rubella antibodies were all 100%, and the GMCs were 1 700 mIU/ml, 2 293 U/ml and 107 IU/ml, respectively. Both the antibody positive rates and GMCs increased after the booster immunization and the differences were significant. Conclusion Before the booster immunization, the positive rates of the IgG declined, especially the positive rates of mumps and rubella antibodies. But the IgG level of mumps and rubella increased significantly after the booster immunization. Current booster immunization program of MMR vaccine in Changping is effective, which can facilitate not only the elimination of measles but also the control of mumps and rubella. It is necessary to maintain the high coverage of MMR vaccine booster immunization.