The primary goal of this research is to determine the effect of large classes on students' achievement at Yarmouk University. The research addresses two questions: What is the effect of class size on Yarmouk University students’ achievement in the English language (101)? Is there are any statistically significant differences between the large and small group treatment on the achievement test? Large class is defined in this paper as a class of more than 45 students. The researcher used a grammar test as the study instrument. The questions were designed based on the grammar curriculum. The subjects of the study were Yarmouk University EFL students who were learning English as a university requirement. Two classes taught by the researcher, one composed of 45 students and the other of 103 students, took the test. The results revealed a relationship between performance and class size. The experimental group (the smaller class) performed better on the test than the control group (the large class). The results of this study highlight to educators the effect of large class on students’ achievement. Also, this may help in developing a curriculum that caters for large classes. In addition, this may draw the attention of university administrators to limit student numbers in English classes. Finally, this study may encourage the government to increase the university budget which would increase the number of classes.