In order to fulfill social responsibility, one of the goals in science education is to equip students with the competence of scientific reasoning. Nevertheless, psychological studies have found that people in general do not have adequate ability to make scientific arguments in everyday situations. Later studies found that the inadequate ability was associated with the development of personal epistemology. However, the conclusion is drawn mostly from research with adults or adolescents. This study attempted to examine the relation between scientific reasoning in informal contexts and the epistemological perspectives demonstrated by elementary school pupils. Participants of the study were 62 sixth graders who were interviewed to criticize two science-related uncertain issues. Content analysis showed that most children had developed the absolutist form of personal epistemology. Chi-square analyses suggested that the more multiplist view toward the certainty of knowledge and the process of knowing, the better coordination of theory and evidence as well as reflective reasoning. In addition, children's beliefs about the certainty of knowledge, source of knowledge and concept of justification were seemingly consistent across different issues. Nevertheless, content analysis showed that the criteria used to make judgments varied with problem contexts.