This study explored the reliability and validity of teacher assessment under a Web-based portfolio assessment environment (or Web-based teacher portfolio assessment). Participants were 72 eleventh graders taking the "Computer Application" course. The students perform portfolio creation, inspection, self- and peer-assessment using the Web-based portfolio assessment system; meanwhile, the teachers used the assessment tool to review students' portfolios and evaluate their learning performances. The results indicated that: 1) the Web-based portfolio teacher assessment achieved an acceptable level of reliability; 2) the Web-based portfolio teacher assessment, showing a strong level of inter-rater reliability and inner-rater reliability, can be regarded as a reliable assessment method; 3) the Web-based portfolio teacher assessment demonstrated an acceptable level of validity; 4) the portfolio scores were highly consistent with students' end-of-course examination scores, implying that Web-based portfolio teacher assessment is a valid assessment method. Future studies are recommended to gain further insight into the self-built portfolios created by digital tools (e.g. PowerPoint, Word, or Frontpage software), for which effective rubrics and reliability or validity of the assessment may be also provided. (Contains 6 tables and 1 figure.)