The purpose of this study is to evaluate the usability of Social Media Classroom (SMC). SMC is a free and open-source web service, developed in 2008. SMC provides teacher and learners an integrated set of social media tools like blog, wiki, forum, chat, and social bookmarking. The usability of this online learning environment was evaluated by cognitive walkthrough which is one of expert evaluation methods. 3 experts who are experienced in human-computer interaction and online learning environments were participated to the evaluation process. Results were presented by analyzing experts' usability reports. Besides, opinions of students' who used SMC for one term, gathered by reports and analyzed. Opinions of students used for supporting the experts' reports.