This article considers the problem of evaluating the reliability of systems subject to common-cause failures. The existing efficient decomposition and aggregation approach for common-cause failures analysis generates and solves a number of reduced reliability problems separately and then aggregates results of those problems based on the total probability theorem to generate the entire system reliability. We propose an enhanced decision diagram-based analysis method, which requires less computer storage and is more efficient as compared with the existing efficient decomposition and aggregation solution by generating a single compact diagram to model all the reduced reliability problems sharing their isomorphic sub-decision diagram. By using multiple-valued variables to encode common-cause failures, the decision diagram-based method offers a simple and straightforward model evaluation process that automatically implements the results aggregation process of the efficient decomposition and aggregation method. Application and advantages of the proposed method are illustrated through detailed analyses of an example computer system subject to common-cause failures.