A RTLinux-based real-time SERCOS monitor is designed and realized to listen to the data message sent from control unit (master) to drives ( slaves) . After analyzing these data according to the SERCOS protocol, whether these data messages monitored are correct or not could be judged. The basic framework of the real-time monitor program and the implementation of its major functions are given, and the experimental results of the cyclic and noncyclic transmission data monitored is also presented.