Reducing accident severity is an effective way to improve road safety. In this article, a novel multiobjective particle swarm optimization (MOPSO)-based partial classification method is employed to identify the contributing factors that impact accident severity. The accident dataset contains only a few fatal accidents but the patterns of fatal accidents are of great interest to traffic agencies. Partial classification can deal with the unbalanced dataset by producing rules for each class. The rules can be evaluated by several conflicting criteria such as accuracy and comprehensibility. A MOPSO is applied to discover a set of Pareto optimal rules. The accident data of Beijing between 2008 and 2010 are used to build the model. The proposed approach is compared with several rule-learning algorithms. The results show that the proposed approach can generate a set of accurate and comprehensible rules, which can indicate the relationship between risk factors and accident severity.