The purpose of this study was to explore the effects of a personal frequency modulation (FM) system on the on-task behavior of a primary school student with multiple disabilities. The research method was a withdrawal design of the single-subject research, using an FM system as the independent variable, and the on-task behavior of a research object as dependent variables. The main findings of the study were as follows: using an FM system could increase on-task behaviors, including eyes turning toward source of sound, body turning toward source of sound, when moving around in the class. Finally, teachers agreed that the results of the FM system shawed its obvious merit, and could accept the FM system.