In an educational milieu student learning outcomes are directly related to teacher professional identity (TPI) i.e. improvement in the TPI will bear a direct positive effect on the learners’ academic achievements. Current study focuses on the development of TPI of English as a Foreign Language Teacher at English Language Institute (ELI) of a Saudi Arabian university through an in-service Cambridge English Teachers (CET) Professional Development (PD) program (CET-PD). Five determinants of TPI - Knowledge of Teaching Context (KCT), Collegial Collaboration (CC), Teaching Practices (TPs), Teacher Self-Efficacy (TSE), and Teacher Agency (TA) were studied before and after the PD program. Retrospective pretest-posttest research design was employed for addressing the research question: whereas responses on the five determinants were elicited from 120 participants through a self-administered questionnaire before and after the CET-PD program. Due to non-normality of data, a non-parametric statistic test-Wilcoxon signed Rank test was employed to analyze the collected data using SPSS. Results of the study revealed that three determinants of TPI - KCT, TSE, and TPs exhibited larger differences; whereas, for CC the differences were moderate and for TA the differences were minimal. By and large, due to in-service CET- PD program TPI exhibited improvement. The results of the study will be beneficial for teacher trainers to focus more on the teachers’ awareness of the learners’ and institutional contextual knowledge in a culture embedded in conservative norms. This study is a part of the quantitative phase of an ongoing Ph.D. project which employs mixed method convergent design.