Teacher effectiveness is regarded as a significant issue in the field of language teaching which has recently received experts’ attention in SLA research. However, no valid instrument has been developed to precisely evaluate the attitudes of Iranian EFL students and/or teachers towards different aspects of teacher effectiveness in the classroom. To do so, in the first phase of this study, a conceptual framework was generated after a comprehensive review of the related literature, and the first draft of the newly-developed EFL teacher effectiveness questionnaire with 47 items was developed and validated with 224 participants. To identify the main components of the questionnaire, an Exploratory Factor Analysis followed by a Confirmatory Factor Analysis was carried out to measure its construct validity. Consequently, the final draft of the questionnaire comprised 42 Likert-point items. In the second phase of the study, the participation of 322 EFL students and 50 EFL teachers was demanded to investigate the effect of modular instruction on EFL teacher effectiveness in the real classroom context. The research results suggested that (1) Iranian EFL learners had a highly positive attitude towards teacher effectiveness constructs such as teacher behavior, attitude, rapport, interest, and effective teaching practice, and (2) the modular instructions had a positive effect on EFL teacher effectiveness. The findings had several pedagogical implications for EFL teachers, teacher educators, and materials developers as the knowledge of teacher effectiveness and modular instruction could positively affect the nature and conditions of language teaching and learning. Manuscript profile