This study explored the reliability and validity of Web-based portfolio self-assessment. Participants were 72 senior high school students enrolled in a computer application course. The students created learning portfolios, viewed peers' work, and performed self-assessment on the Web-based portfolio assessment system. The results indicated: 1) there was a high consistency level between students' two assessment results; 2) the self-assessment results and teacher-assessment results were highly consistent without significant differences; 3) the results of self-assessment and end-of-course examination were highly consistent, implying that Web-based portfolio self-assessment can faithfully reflect learning achievements, i.e. it is a reliable and valid assessment method. (Contains 8 tables and 2 figures.)