The face-to-face computer-supported collaborative learning has emerged as an important strategy to elementary students' learning. Few studies have explicitly incorporated the structured peer tutoring to the synchronous environment in mathematics. This study was aimed to explore the effects of the synchronous peer tutoring system on children's mathematics learning. In the project, there were 88 10-11-year-old students who peer tutored each other in mathematics in the face-to-face online environment for a year. Compared to the control group, students in the experimental group had significant gains in mathematics learning, especially in the arithmetic and application types of questions. This study demonstrated the positive effects of peer support via the online synchronous learning on students' self-concept and attitudes toward mathematics learning. The results indicated that the longer the vulnerable pupils engaged in peer tutoring online, the more they benefited from the process. The finding suggests that students demonstrate different mathematics reasoning skills when they are paired with peers at different levels of ability. These findings demonstrate that the synchronous peer tutoring system is an effective tool to enhance elementary students' learning in mathematics, as well as promote positive self-concepts. (Contains 5 tables and 2 figures.)