Web based learning is considered as a breakthrough in the teaching of writing skill to the pre-service teachers at University of PGRI Semarang, Indonesia. The students should write argumentative, persuasive, and descriptive essays. This research offers significant contribution in term of the impact of web based learning on writing skill of English as Foreign Language (EFL) learners. There were 156 students who joined ODF but only 63 students (40%) were eligible. The objectives were to find its effectiveness for improving students' writing skills. Content, organization, vocabulary, syntax, mechanics, and styles of students' writing were assessed by Brown's writing rubric. Interview was conducted to find out the area of students' difficulties. The results show improvements of means score from 76.47 increased into 80.68 during post-test. The forum has significant influence on improving writing skill (t-test 2.170 > t-table 1.999). p<0.05, df = 62 as the students submit six to nine postings. It indicates that the students need more practices of writing. Among six writing components, syntax received the most improvement though few students still have problems with conjunctions, passive voice, and conditional clauses.