The most prominent factor influencing students' learning, in any instructional program, is the textbook (Litz, 2001). Due to such an importance, the textbook is regarded as the ELT program heart and an essential constituent to many EFL/ESL classrooms. Therefore, textbooks should be evaluated and selected as carefully as possible to avoid any adverse consequences (Sheldon, 1988). Thus, this study intends to investigate the effectiveness of Iranian first grade Junior High School English textbook from the teachers' points of view. To achieve the goals of study, 100 male and female EFL teachers from junior high schools were randomly selected. They were given a researcher-designed, 5-point Likert scale questionnaire including 45 items for investigating their beliefs on the effectiveness of the textbook regarding eight main factors: grammar, general features, listening, vocabulary, speaking, comprehension, writing, and teacher guide. In addition to the questionnaire, a semi-structured interview conducted for providing qualitative information about the EFL teachers’ beliefs based on the questionnaire and knowing the participants’ underlying reasons for their beliefs they offered in written questionnaire. The results of study revealed that female teachers, by getting higher mean scores in some factors such as grammar, vocabulary, and writing, showed that they are more satisfied with the effectiveness of these parts, while male teachers were more satisfied with general features, listening, speaking, comprehension, and teacher guide.