This study explored the effects of peer teaching on mathematics academic achievement of the undergraduate students in Oman. The sample of this study composed of (32) undergraduate female students enrolled in the course, "Mathematics for Social Sciences I" in Mathematics and Sciences Unit in Dhofar University in spring semester 2014-2015. The sample of the study was divided randomly into two groups, the first group was selected as experimental group (16) students and the other group was selected as control group (16) students. Both groups studied the same curriculum, same materials, and both groups had been taught by the same instructor. Data were collected by using, Mathematics Achievement Test (MACT) which was prepared by the researchers. In order to analyze data, researchers used mean, standard deviations, and Analysis of t-test. The result of the study found that peer teaching strategy is an active tool to increase the mathematical achievement. The study recommended the instructors and college professors to use peer teaching in order to improve the performance of the students in mathematics.