The paper reports a research and development (R&D) type of study which devised a teacher’s weblog for portfolio assessment in an English as a foreign language (EFL) writing class. As previous research suggests, portfolio assessment is effective to improve students’ writing performance (Nezakatgoo, 2011), and has a positive effect on language learning and writing ability (Taki, 2011). A model of weblog with facebook login was designed and used as a virtual medium in which students could post their writing works; the drafts, the revised and the final versions. In this way, their works are accessible not only for the teacher and their peers but also the public as well. The teacher and students could also write comments as feedbacks for improvement of the drafts. The students taking Writing 3 at English Education Department of Sultan Agung Islamic University (UNISSULA) Semarang participated as the subjects of the study. At the end of the semester, the data on the effectiveness of the model were collected by a four scale of Likert-like questionnaire and were analyzed descriptively. The students found that the model has many advantages over paper-based portfolio as it is more practical, easily accessible and encourages them to better prepare the writing.