Abstract: Hard real-time distributed systems play a vital role in many areas, such as national defense, flight control systems, space shuttle and aircraft avionics. Predictability is one of the most important characteristics. Based on a hard real-time distributed system prototype, latency factors are analyzed and tested, which affect the system predictability. One scheme for testing real-time operation system overheads is presented. A computing model of maximum response time is set up, and one application example is given. The experimental results illustrate the correctness of the work in this paper. They are significant for the design and time correctness verification of hard real-time distributed systems.