Building on Carey and Smith's work on epistemology of science, this study characterized elementary students' scientific epistemology from the role-of-idea and theory-building perspective. The participants included 102 elementary students in Hong Kong. Open-ended questions and coding schemes were developed to examine and characterize their epistemic understanding of science. Four key dimensions were identified: role of idea, theory-fact understanding, theory revision and creation, and social process of scientific progress. The general pattern showed that students' views of scientific inquiry ranged from seeing it as a mechanical process involving concrete materials and sets of scientific skills, to viewing it as an idea-driven process comprising collective theory-building and knowledge creation. The analysis showed that these students generally had a limited understanding of the role-of-idea and theory-building in science. Quantitative analysis indicated that this conceptualized epistemology of science was an important predictor of students' academic performance in science. Implications for science education and future research are discussed.