Abstract: The Fieldbus Foundation protocol is based on a centralized communication media access control mechanism, where a link active scheduler controls all messages transfer between different field devices on a bus according to a schedule time list pre-defined. Firstly, since the rate monotonic algorithm did not consider the schedule for the tasks whose periods are same, the paper provides the algorithm of extended rate monotonic to assign the priority level for all communicating periodic messages. Secondly, the approach how to build a schedule time list based on the extended rate monotonic arithmetic is proposed, and the condition judging the validity of the schedule time list built is given by analyzing the maximum waiting time, in order to guarantee the real-time communication of the periodic messages. Finally, the extend rate monotonic algorithm and its application in building the FF schedule time list are showed by a example of application.