This research explored the influence of a Web-based portfolio assessment system on students' performances. The methodological procedure adopted was to have the experimental group use the system, with the control group using conventional assessment. The study subjects were junior high school students of two computer classes. The experimental results revealed that the use of the system has significant positive influence on students' performances. According to estimated effect size, the most significant indicators were reflection, self-assessment, continuous improvement, goal setting, problem solving, data gathering, work and peer interaction. However, peer-assessment performance was not enhanced significantly. Therefore, one recommendation was to reduce peer-assessment and instead offer specific illustrations to the students as well as the opportunity to drill.