The effects of students' preconception, self-checking, and effort on their responses to anomalous data, the degree of cognitive conflict by anomalous data, and conceptual change in studying 'the conservation of mass during the change of states' were studied. A preconception test and a test of response to anomalous data were administered to 159 sixth graders. Among them, 81 students whose preconceptions were consistent with the presented initial theory were selected. Immediately after they had read a scientific explanation, their conceptual change were examined with a researcher-made conception test. The results indicated that students who had the preconceptions consistent with the initial theory exhibited more 'rejection' and less 'theory change' patterns than the others. Significantly more cognitive conflict was also induced for the students who had the preconceptions. More 'theory change' patterns were found for the low self-checking students, and less 'rejection' and more 'theory change' patterns were found for the students investing high effort. However, Mann-Whitney U test indicated no significant differences in the degree of cognitive conflict by the levels of self-checking and effort. In the conception test scores, there were no significant differences by students' preconceptions and by levels of self-checking and effort.본 연구는 서울대학교 사범대학 발전기금 지원으로 이루어졌음