This study examines the effect of a Web-based portfolio assessment system on the performances of students undertaking project-based learning (PBL). The research targets were 60 students from two grade-8 classes taking senior high school computer courses. The experimental group comprised 30 students, who used the system to perform PBL and assessment in one class. The control group comprised of 30 students who employed conventional assessment for PBL of another class. Experimental results indicate that the system has no significant effect on student achievement, but had a statistically positive effect on self-perceived learning performances. In addition, teacher-assessment and self-assessment of project achievements produced different results. (Contains 2 figures and 9 tables.)