The purpose of the present study was to design reflective writing mechanisms in a web-based portfolio assessment system and evaluate its effects on self-regulated learning. Participants were two classes of juniors majoring in data processing and taking a course called "Website design" at a vocational high school in Taiwan. One class was randomly selected and assigned as an experimental group (41 students) reflecting on learning processes through a web-based portfolio assessment system, whereas the other class was assigned as a control group (41 students) reflecting on learning processes through a paper-based portfolio. The result revealed that students who were highly satisfied with online reflective writing mechanisms significantly outperformed students who were less satisfied with online reflective writing mechanisms in self-regulated students reflecting on learning processes through a web-based portfolio assessment system significantly outperformed students reflecting on learning processes through a paper-based portfolio in self-regulated learning.