This study examines the reliability and validity of Web-based portfolio self-assessment. The research sample consisted of seventy-two students taking an ”Introduction to Computers” course in the second grade of a vocational high school. The students used a Web-based portfolio assessment system to create their portfolio, emulate peers' portfolios, and perform self-assessment. The results demonstrate that there exists a high consistency between the two self-assessments. There exists a high consistency and no significant difference between student self-assessment and teacher assessment. The consistency between peer-assessment and test score was high, implying that the Web-based portfolio self-assessment may reflect learning achievement. In short, Web-based portfolio self-assessment has adequate reliable and validity.