The fuzzy logic Mamdani model is a fuzzy reasoning system that can be used to calculate values based on various criteria for evaluating teacher performance. In this study, three fuzzy sets were used to determine the level of success of teachers in teaching through online media in one of the vocational schools in North Sumatra: the teacher set, the value set, and the success rate set. The teacher set is divided into three criteria: bad, sufficient, and good teachers. The set of values is also divided into three categories: bad, sufficient, and good. Furthermore, the set of success rates is divided into three categories: low, medium, and high success rates. The goal of this study is to use the mamdani fuzzy logic method to assess the level of success in teaching teachers using online media. According to the study's findings, the mamdani method could be used in this situation.