The research compared students' summary writing ability before and after they were taught through blog, a new medium or tool for written communication and interaction in many different languages around the world. The research design is a kind of one group pretest posttest. Participants were 33 first-year students who studied EN 011 course (English in Action) in the first semester of the academic year 2012 at Bangkok University. They were divided into six groups. Five or six students in each group created a blog; www.blogger.com, and they worked together for fourteen weeks to produce six pieces of summary written work. Each member in the group worked through providing comments, editing and revising on the blog until the group got a final summary paper and submitted that to the teacher for grading. The instruments used in this study were (1) two summary writing tests (2) a questionnaire surveying students' attitude toward learning through blogs, and (3) postings on blogs to reflect their learning experience. The results revealed that after the students worked together on weblogs, their English summary writing mean score of the posttest was higher than that of the pretest, and they had positive attitudes towards using weblogs in learning. Regarding cooperative learning experiences through using weblogs, most students thought that it was interesting, a new experience to work with their friends on the weblogs. (Contains 2 tables.)