This study aimed to develop Al-Azhar University English language instructors to investigate how a proposed program based on the inquiry approach affected their inquiry-based teaching performance. The study utilized a list of the inquiry-based teaching performance skills, based on which an observation checklist was developed to assess participants' inquiry-based teaching performance. Furthermore, the results of the quantitative analysis of the observation processes were obtained from semi-structured interviews. 16 university English language instructors at the Faculty of Education in Al-Azhar university participated in the study and were trained on the inquiry approach with its five Es model. The results of the study showed the positive effects of using the 5E model on the participants' inquiry-based teaching performance in terms of questioning, presentation, engagement, and assessment. The use of the inquiry approach improved the university English instructors' inquiry-based teaching performance as demonstrated by the qualitative analysis of the interviews and results of the observation checklist.