Abstract: The widespread use of test scores for different educational and social decision making purposes has made the washback effect of tests a distinct educational phenomenon (Cheng, 1997).The high school third grade final exam in the general educational system of Iran has for long been a high stake test designed to assess the achievement of high school graduates in different school subjects. The present study aimed to investigate the washback effect of this nation-wide exam on EFL teachers’ teaching methodology, assessment procedures, and attitudes towards different aspects of the educational system. For this purpose, a researcher made, validated questionnaire was administered to 160 EFL teachers. The results indicated that the third grade nation-wide final exam adversely affects EFL teachers’ teaching methodology and increases teaching to the test effect quite noticeably as they try to teach according to the content and format of the test. The results further showed an even stronger negative effect of the exam on EFL teachers’ assessment procedures. However, the teachers’ attitude towards different aspects of the educational system was not found to be as strongly affected as the other two variables. The findings of the study are of importance for testing and assessment bureaus in charge of extensive high stake tests development. Moreover, raising teachers’ awareness of the drawback of teaching-to-the test effect of such a high stake test might help them improve their teaching and evaluation practices.