Abstract: Objective To understand the incidence pattern of adverse events following immunization (AEFI) in Inner Mongolia in 2011 and evaluate the performance of AEFI surveillance system. Methods The incidence data of AEFI in Inner Mongolia during the year were collected through the surveillance system of adverse event following immunization to conduct statistical analysis with Excel and the descriptive epidemiological analysis. Results A total of 594 AEFI cases were reported. More cases occurred in males than in females with the ratio of 1.22: 1. In these cases, 547 were general reactions after immunization, accounting for 92.09%, 37 were abnormal reactions after immunization accounting for 6.23%. The incidence peaks were during April-July and during October-December. Totally 220 cases occurred in children aged 1 years. accounting for 37.04%, and 238 cases were caused by the immunization of diphtheria, tetanus and acellular pertussis (DTP) combined vaccine, accounting for 40.07%. The cases (493) occurred within 24 hours after immunization accounted for 83%, the cases reported within 48 hours accounted for 85.86% and the cases surveyed within 48 hours, accounting for 98.04%. Conclusion The analysis indicated that the performance of AEFI surveillance system in Inner Mongolia was good and the sensitivity of the system was obviously higher in 2011 than in 2010,but the reporting rate within 48 hours should be increased.