This study reports on how students construct new language knowledge by indirect feedback in web-based collaborative writing. Indirect feedback (text organization, reader-based perspectives, and clarity of purpose) encourages students to negotiate meaning instead of merely copying peers' direct feedback on grammatical corrections. According to the pre-test, 54 students were divided into the more- and less-proficient groups with 27 students in each group. The pre- and post-tests, students' log files recorded in the system, and an open-ended questionnaire were used in this study. The results indicate that the more-proficient students were able to provide indirect feedback to elaborate their thoughts. They further made decisions on accepting or rejecting their peers' or the teacher's suggestions while they integrated previous knowledge with new information they had learned from their peers and the teacher, which lead to new ideas generated in the final drafts. In contrast, the less-proficient students had predominantly copied their peers' direct feedback and corrected grammatical errors as they were not capable of producing indirect feedback. After engaging in more learning with the help from the more-proficient peers, the less-proficient students made more writing progress with indirect feedback than the more-proficient students. Through indirect feedback to construct new language knowledge, web-based collaborative writing fostered the more- and less-proficient students to learn from each other by negotiating meaning for text improvement.