Reflection is a key component of teacher quality assessment and a venue for professional development. Among many tools for teacher reflection, keeping blogs as a user-friendly, technology-enhanced tool has recently come to the forefront in teacher education. To contribute to the body of literature on the potentiality of blogs for teacher professional development, this study sought to investigate the use of blogs by Iranian English as a foreign language (EFL) teachers to provide a perspective on the themes in their reflection-on-action and the degree of criticality of their reflection. To this end, 32 male and female in-service EFL teachers, teaching at different language institutes, voluntarily agreed to post their classroom experiences to a blog over a 3-month period. Content analysis of the reflection tags resulted in the identification of three main reflective categories, each including a number of themes: (a) teacher-oriented category (e.g., teachers' sharing emotions, classroom management, teaching methods and techniques, use of first language [L1], and instructional ethics); (b) learner-oriented category (e.g., learners' classroom participation, language skills, motivation, and emotions); and (c) context-oriented category (e.g., assessment system, textbooks, classroom facilities, and teachers' working conditions). Moreover, the levels of reflection embedded into the tags were explored using a modified version of a reflection model proposed by Lee (2005). The findings revealed that teachers were more critical (71%) than descriptive (29%) in their reflections. These findings have implications for teachers' reflection and their professional development.