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This study aims at investigating the effect of dynamic assessment (DA) on L2 writing achievement if applied via blogging as a Web 2.0 tool, as well as examining which pattern of interaction is more conducive to learning in such an environment. The results of the study indicate that using weblogs to provide mediation contributes to the enhancement of the overall writing performance, vocabulary and syntactic complexity, and quantity of overall information presented in a single paragraph. That is to say, DA procedures are applicable via Web 2.0 tools and are advantageous to L2 learners’ writing suggesting that L2 practitioners and instructors should actively consider the integration of Web 2.0 technology into L2 education system using DA. Moreover, the collaborative pattern of interaction as compared to expert/novice, dominant/passive, and dominant/dominant patterns is found to be more conducive to fostering writing achievement in the asynchronous computer-mediated communication environment.