Few studies have examined the effects of using mediation tools (e.g., collaborative dialogs and online searches) on graduate students' academic writing in a computer-mediated collaborative writing environment. This study investigated what types of co-constructed writing knowledge the graduate students generated from using the mediation tools to solve writing problems, and to what extent the co-constructed writing knowledge facilitated each student's academic writing. Thirty-six Taiwanese graduate students, who studied English as a foreign language (EFL), volunteered to participate in an online collaborative writing program featuring developing argumentative essays. Data collected in this study include the students' collaborative dialogs, online search logs, open-ended questionnaires, and the students' collaborative and individual writing texts. The results show that the students using the mediation tools could generate two types of co-constructed writing knowledge in terms of language forms and skills of expressions, and they outperformed those who did not, in aspects of holistic performance, communicative quality, text organization, argumentation, linguistic accuracy, and linguistic appropriateness. It is concluded that students may co-construct writing knowledge to solve writing problems and improve both collaborative and individual writing through effective use of the mediation tools available in an online collaborative writing environment.