Textbooks play a very crucial role in the process of language teaching and learning. The present study carries out an evaluation of two series of ELT textbooks used for teaching English language in Iranian high schools from 1970 to the present. For this purpose, Tucker's (1975) textbook evaluation model is employed. The results suggest that one of the main factors for the students' achievement in English language is the ELT textbooks. The researchers suggest that in the textbooks, there should be enough opportunity for the learners to practice the language they are learning communicatively.