The advancement of computer technology and expansion of the Internet has an increasing impact on writing instruction. The modes of peer feedback have shifted from traditional peer feedback to online peer feedback. This study investigated the effects of peer feedback activity through blogs on students’ writing ability and examined their attitudes towards peer feedback activity. The research was conducted using a single group pretest-posttest design. Blog, the website, was used as a medium for peer feedback activity. Participants were 34 second-year students who studied EN 013 course (English for Expressing Ideas) in the first semester of the academic year 2012 at Bangkok University. Two writings tests and a questionnaire were used as instruments for data collection to acquire information. The results revealed that students’ writing scores on the pretest and posttest were significantly different. It can be concluded that peer feedback activity through blogs had a significant role to play in improving students’ writing skill. The students also expressed positive attitudes towards the value of peer feedback activity.