Cycloid reducer belong to the latest generation of planetary gear trains. Compared with conventional speed reducers, the cycloid reducer cover a wider range of transmission ratios, have a higher load-carrying capacity, are smaller, exhibit a smoother running and a good efficiency. The cycloid speed reducer consists of cycloidal plate gears, housing gear, input shaft, output pin and shaft, and eccentric bearings. These characteristics make it attractive for industrial applications, especially for robotics applications, machine tools and linear axis positioning in assembly machinery. However, the cycloid speed reducer has a eccentric rotating motion, so it has a disadvantage for vibration. In this paper, a multi-body dynamic model is developed for a cycloid speed reducer and the dynamic behaviors of the reducer are investigated.