The purpose of this study was to develop and validate an online contextualized test for assessing students' understanding of epistemic knowledge of science. In addition, how students' understanding of epistemic knowledge of science interacts with learner factors, including time spent on science learning, interest, self-efficacy, and gender, was also explored. The participants were 489 senior high school students (244 males and 245 females) from eight different schools in Taiwan. Based on the result of an extensive literature review, we first identified six factors of epistemic knowledge of science, such as status of scientific knowledge, the nature of scientific enterprise, measurement in science, and so on. An online test was then created for assessing students' understanding of the epistemic knowledge of science. Also, a learner-factor survey was developed by adopting previous PISA survey items to measure the abovementioned learner factors. The results of this study show that: (1) by factor analysis, the six factors of epistemic knowledge of science could be grouped into two dimensions which reflect the nature of scientific knowledge and knowing in science, respectively; (2) there was a gender difference in the participants' understanding of the epistemic knowledge of science; and (3) students' interest in science learning and the time spent on science learning were positively correlated to their understanding of the epistemic knowledge of science.