Background: Researchers have indicated that assessment practices and methods should support learners' construction of meaningful understanding of knowledge. Understanding students' conceptions of assessment will enable us to construct more realistic, valid and fair assessments. Learners' conceptualization of assessment would be imperative to serve as an essential reference to evaluate their learning progress. Purpose: This study evaluated and compared the Singaporean and Taiwanese middle school students' conceptions of science assessment. Within-country gender comparisons were also explored. Sample: 424 Taiwanese and 333 Singaporean eighth graders were invited for a cross-country comparison. Design and methods: The participants completed a questionnaire named Conceptions of Science Assessment. Results: The findings showed that the summative assessment dominates in the Taiwanese classrooms, while formative and summative assessment are usually perceived in Singaporean classrooms. The Singaporean students had a greater tendency than their Taiwanese counterparts to perceive the purpose of assessment as a way of reproducing knowledge, and the formative assessment as improving learning, problem-solving, and critical judgment. No gender differences were found among either the Singaporean or the Taiwanese students. Conclusions: Educators in both countries should provide learners with more opportunities to experience process-oriented science assessment activities and de-emphasize the usage of examinationoriented practices to achieve the sophistication of conceptions.