As part of a large-scale project, this study investigated the differences between satisfied and dissatisfied Iranian junior secondary school English teachers in terms of their job performance. To this end, 64 Iranian English teachers and 1774 of their students completed a validated questionnaire specifically developed to investigate EFL teachers' job performance. The results indicated that satisfied teachers significantly differed from their dissatisfied counterparts in terms of their job performance suggesting that the observed differences in their job performance might have mainly been caused by the degree to which they were satisfied with their profession. The results also showed a significant difference between students' evaluation of their teachers' job performance and the teachers' self-evaluation of their job performance. Furthermore, through a semi-structured interview, the most crucial factors contributing to poor job performance of dissatisfied Iranian secondary school EFL teachers were identified to be an insufficient subject and pedagogic knowledge, unequal attention to individual students, lack of professional commitment, interpersonal relationship problems, and demotivation.