The purpose of this study was to investigate the effective teaching strategies to reform learners' erroneous laws, called Fu's. One hundred eighty undergraduate subjects had a few ru's on buoyancy. Four groups called A (GA), B (GB), C (GC) and D (GD) were formed. GA were asked to solve the problems with estimates of solving based on the relevant ru's and to judge before reading the text of the nature of buoyancy, what laws they had used on that occasion. After reading the text, they were imposed to rate the understanding and unexpectedness on the content of the text, and to take the post-test for an evaluation of reform of ru's. GB were given the same tasks as GA except the judgement task. GC were not given any task before reading, and GD were given the post-test only. In the post-test, GA and GB made better marks than the others. But the law acquired by GA showed a tendency to become refined in comparison with GB. These results corresponded with the difference of the unexpectancy rating values between the two.