This study attempted to map EFL teachers' interpersonal behavior as perceived by students, its relationship with the students' English language achievement, and to examine if it could predict students' English language achievement. The samples of the study selected 353 grade 9 students from Arba Minch and Chamo Preparatory and Secondary schools in Ethiopia. Data were collected through interpersonal behavior questionnaire and English language achievement test. In order to map teache interpersonal behavior, mean and standard deviations of the QTI dimensions were used while multiple regression was used to examine the correlations between teachers' interpersonal behavior and English language achievement test scores. Furthermore, mult regression was used in order to examine the degree to which EFL teachers' interpersonal behaviors predicted their students' English language achievement score. The findings indicated that understanding was perceived as interpersonal behavior which occ often in English language classes followed by leadership, strict and student responsibility/freedom behaviors respectively. Divergently, uncertain behavior was found to be least perceived behavior. With respect to the major dimensions of interpe the result showed that teachers in the sample areas are less dominant but more cooperative over the communication process with their students. Concerning associations, this study found positive and significant correlations between students and their teachers' leadership and understanding behaviors. Regarding the dimensions, both proximity and influence were correlated positively and significantly with students' English language achievement. It was also found th together explained 6.3% of the variances in the students' English language achievement scores. Pertaining to the relative contribution of each independent variable, understanding, followed by leadership interpersonal behavior was found to be the most powerful and significant variable on the students' English language achievement while dimension wise, only influence was determining students' English language achievement significantly.