This study sought to investigate dynamic assessment (DA) - an assessment approach that embeds inter- vention within the assessment process and that yields information about the learner’s responsiveness to this intervention - and the writing performance of the second language (L2) learners in Web 2.0 contexts. To this end, pre and post-treatment writings of 45 participants were analyzed to examine the impact of asynchronous collaborative computer mediation and face-to-face collaborative mediation on L2 learners’ writing performance. Three textual features of syntactic complexity, vocabulary complexity, and quantity of the overall information conveyed in the learners’ pre and post-tests served as the basic units of analy- sis. The findings of the present study indicated that using blogging as a Web 2.0 tool to provide mediation contributed more to the enhancement of the overall writing performance. Moreover, the asynchronous collaborative computer mediated group, as compared to the face to face mediated group, showed signifi- cant improvement in the vocabulary complexity, syntactic complexity, and quantity of overall infor- mation presented in a single paragraph. The findings of the present study also revealed that DA proce- dures were applicable via Web 2.0 tools and were advantageous to L2 learners’ writing performance sug- gesting that L2 practitioners and instructors should dynamically consider the integration of Web 2.0 tech- nology into L2 writing courses. Manuscript profile