College students often experience difficulties in solving physics problems. These difficulties largely result from a lack of conceptual understanding of the topic. The processes of conceptual learning reflect the nature of the causal reasoning process. Two major causal reasoning methods are the covariational and the mechanism-based approaches. This study was to investigate the effects of different causal reasoning methods on facilitating students' conceptual understanding of physics. 125 college students from an introduction physics class were assigned into covariational group, mechanism-based group, and control group. The results show that the mechanism-based group significantly outperformed the other two groups in solving conceptual problems. However, no significant difference was found in all three groups performance on solving computational problems. Speculation on the inconsistent performance of the mechanism-based group in conceptual and computational problem solving is given. Detailed analyses of the results, findings, and educational implications are discussed. (Contains 6 figures and 2 tables.)