In order to contribute to the line of inquiry on the use of Web 2.0 technology in Second Language (L2) learning, the current research employed a quantitative research method to probe the contribution of blog-mediated writing instruction to developing the writing skill and the ideal writing self of Iranian English-as-a-Foreign Language (EFL) learners. In so doing, a sample of 47 Iranian EFL learners in a private language institute were recruited as the subjects of this research. These students were then divided into two classes and were randomly assigned to an experimental class (N = 24) and a control class (N = 23). Concerning the research intervention, the students of the blog group used blogs in their writing instruction while those in the control group received the conventional, inside-the-class writing instruction. The Ideal L2 Writing Self Scale was given before and after the intervention as the pre-test and the post-test, respectively. The results from ANCOVA analysis indicated that although both groups progressed in both writing performance and ideal writing self scores, the learners of the blog group outperformed those of control group, suggesting that the blog-mediated instruction significantly contributed to improving the writing performance (F= 9.78, p = 0.003, partial eta squared = 0.18) and ideal L2 writing self of the EFL students (F= 11.678, p = 0.001, partial eta squared = 0.210). These results may offer important implications for EFL writing research and pedagogy. Overall, the integration of blogs in L2 pedagogy might be effective type of blended learning approach which can result in better linguistic and affective outcomes.