The purpose of this study was to investigate the relationship of students' understanding of science knowledge, attitude and decision making on socio-scientific issues (SSI), especially on the issues of nuclear energy in Korea. SSI-focused instructions were developed to encourage students to understand and reflect on knowledge, attitude and decision making on nuclear energy in the current society. Eighty-nine students attended the instruction and participated in pre and post questionnaires to understand their understandings of nuclear energy. In this study, science knowledge was categorized into content and contextual knowledge, attitude consisted of images, safety, risk, potential, benefits and future roles, and decision making section included preference and alternative about lifetime extension of nuclear power plant. The results of questionnaires were analyzed by correlation, cross-tabulation and regression. As a result, while students' understandings of science knowledge were significantly improved throughout the instruction, they maintained similar attitude and decision making on the issue. Regarding the relationship of the three domains, attitude showed some degree of connection to decision making whereas science knowledge did not show a significant relationship to decision making. This finding challenges SSI teaching in content-based science curriculum and classroom. Reflection and implications on the way of teaching SSI in the classroom were discussed further in this paper.