Employing an explanatory sequential design, this study investigated the effects of a blog-mediated writing course on L2 students’ writing motivation, self-efficacy, and self-regulation. A number of 46 Iranian EFL learners from 2 intact university classes were recruited as the participants and were randomly assigned into the control group (n = 21) and the experimental group (n = 25). Over a 16-week university semester, the control group was taught using traditional writing instruction, whereas the experimental group was taught using a blog-mediated writing course. Data were collected through administration of 3 scales, measuring L2 writing motivation, self-efficacy, and self-regulation. Also, to reveal a more comprehensive understanding of blog-mediated writing instruction, a set of semistructured interviews were conducted with a number of participants in the experimental group. ANCOVA analyses and thematic data coding were conducted for the quantitative and qualitative data analyses, respectively. Findings revealed that integrating blogs into EFL writing instruction helped the experiment group to have more writing motivation and writing self-regulation than the control group that only had received regular in-class instruction. However, the blog-mediated writing course decreased the writing self-efficacy of the EFL students. Findings offer theoretical and pedagogical implications for L2 writing instruction.