This communication uses the concept of dynamic equilibrium to derive a relation with which the actuator forces of a manipulator with a closed kinematic chain can be calculated like those of a manipulator with an open chain. Although the result is equivalent to Luh and Zheng's , the present method provides the physical insight of dynamics of the manipulators with a closed kinematic chain. This relation also leads to the dynamic equations of motion with a minimal dimension for a closed kinematic chain. The semi-direct-drive robot is taken as an illustrative example. The numerical results verify the theory. The computational complexity of the inverse dynamics of this robot is also discussed.