This study aimed to develop university English instructors' teaching performance using the inquiry approach. As such, the study investigated the effect of a proposed program based on the inquiry approach on developing Al-Azhar University English instructors' teaching performance. The study utilized a list of the teaching performance components related to the inquiry-based approach, based on which an observation checklist was developed to assess participants' teaching performance. furthermore, semi-structured interviews were conducted to substantiate the results of the quantitative analysis of the observation processes. Sixteen university English instructors at the Faculty of Education Al-Azhar university participated in the study and received the training program based on the inquiry approach with its 5Es model. The results of the study revealed the positive effects of the training program in developing the participants' teaching performance in terms of questioning skills, lesson presentation, engagement of students and assessment procedures. Qualitative analysis of the semi-structured interviews supported the quantitative results of the observation checklist that revealed that the use of the inquiry approach has developed university English instructors' teaching performance.