This study investigated the relationship among 68 high school students' scientific epistemological beliefs (SEBs), cognitive structures regarding nuclear power usage, and their informal reasoning regarding this issue. Moreover, the ability of students' SEBs as well as their cognitive structures for predicting their informal reasoning regarding this issue was also examined. The participants' SEBs were assessed with a quantitative instrument; their cognitive structures were assessed through tape-recorded interviews and were further analyzed with the "flow map method"; their reasoning regarding nuclear power usage was assessed with an open-ended questionnaire; and, then, their responses were analyzed both qualitatively and quantitatively. It was revealed that students' beliefs about the justification of scientific knowledge (an aspect of the beliefs on the nature of knowing science) were significantly correlated with their reasoning quality; the extent and the richness of students' cognitive structures as well as their usage of the information processing mode, "comparing," were positively correlated with their reasoning quality. A series of regression analyses further confirmed that students' use of the information processing mode, "comparing," was the most significant factor for predicting reasoning quality, while their beliefs regarding the justification of scientific knowledge was the other important predictor. (Contains 9 tables and 3 figures.)