This study investigated the effects of blog-mediated instruction on English-as-a-foreign language (EFL) learners’ writing performance and anxiety. In addition, it aimed to probe into the EFL learners’ attitudes towards blog-mediated writing instruction. The participants of the study included forty-six Iranian EFL learners from two intact university classes, who were randomly assigned to 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 while the Experimental Group was taught using a blog-mediated writing course. The data were collected through two timed writing tasks, Second Language Writing Anxiety Inventory (Cheng, 2004), and semi-structured interviews. The results indicated that, although both groups benefited from their writing sessions, there was a significant difference in the positive effects of blog-mediated and traditional writing instruction on L2 writing performance, showing that the Experimental Group had a better performance on the posttest writing performance task than the Control Group. The results also revealed that the blog-mediated course reduced the participants’ L2 writing anxiety in the Experimental Group while traditional instruction did not have positive effects on reducing L2 writing anxiety in the Control Group. The data from semi-structures interviews indicated that the interviewees from the Experimental Group were generally positive about the blog-mediated writing course, with little skepticism and negativism echoed about the course. The findings offer significant implications for theory and practice on L2 writing instruction.