Objective: This study aims to investigate the interaction between response rate and class size and its effects on students' evaluation of instructors and the courses offered at a higher education Institution in Saudi Arabia. Study Design: A retrospective study design was chosen. Methods: One thousand four hundred and forty four different courses belonging to all the Colleges (N = 21) located across seven different campuses of University of Dammam (UOD) were considered in this study. All the course evaluation surveys (CES) (N = 168,574) conducted during the academic year 2013-2014 were analyzed to investigate response rate and class size interaction effect on students evaluation of courses and instructors. Results: It is observed that when the class size is at the medium level, the ratings of instructors and courses increase as the response rate increases. On the contrary; when the class size is small, a high response rate is required for the evaluation of instructors and at least medium response rate is required for evaluation of courses. The study suggests that the interaction between response rate and class size is an important factor that needs to be taken into account while interpreting the students' evaluation of instructors and courses. Originality: This research study examined the effect of interaction between the response rate (RR) and class size (CS) on students' evaluation of instructors and courses in higher education. This study will help the policy planners of higher education institutions to understand the response rate required for different class sizes while planning students' evaluation surveys for courses and instructors.