Shared responsibilities such as mentoring, instruction, learner monitoring and classroom management enable the peers to observe, review, reflect on and learn from the overall practical professional expertise of one another through collegial instruction experience. The present exploratory case study has The present exploratory case study has attempted to study collegial teaching as an innovative instruction model (as an alternative to solo-based instruction models) in a General Business English course in SAPco (An automotive part supplier in Iran). To this end, the researcher has mainly tried to concentrate on two expert business English teachers' perceptions concerning their experience of collegial teaching for business English courses, observing their reflections before and after the course, to focus on the possible impacts of this type of instruction on their pedagogical knowledge as language teacher. As a result, as it is indicated in overall reflections of the participant colleagues, collegial instruction is believed to lead toward a more efficient transferability and development of teachers' pedagogical knowledge than what can take place as a result of individually run traditional practices. Moreover, this type of instruction can be a rather cost-effective and timesaving alternative to traditional OJT (on the job training) courses for teacher development authorities and curriculum developers who are concerned about and willing to promote professional development of their teachers.