The development of ocean oil fields are now going to deeper and smaller oil fields. A FPSO (Floating, Production, Storage and Offloading) system is one of the most economical systems for the production of marginal fields and thus is in use worldwide. For the position keeping of a FPSO system, a single point mooring system is usually employed to economize the operation. However, the conventional single point mooring systems such as a catenary anchor leg mooring (CALM) or a turret mooring (TM) become less economical as the water depth increases because the system becomes heavier in order to sustain the large weight of mooring lines. In this paper, we apply a Counterweight Articulated Mooring (CAM) system, which was originally proposed for a mooring system in ice infested water, as the alternative single point mooring system for a FPSO that may be used in a medium-depth water (500 m). Since a CAM system is mechanically much more complicated than conventional systems, we examine how the behaviours of a CAM system in waves can be predicted theoretically. A computer code is developed based on the constraint matrix method proposed by Langley for the analysis of a floating system composed of a number of structures. Numerical results obtained by the developed code are compared with model test data. And the following conclusions are obtained through the present study. (1) The numerical prediction of wave induced responses of a CAM system gives fairly accurate results. (2) The low frequency responses in irregular waves are governed by the low frequency oscillations of a tanker, which should be predicted while properly accounting for the damping force. (3) The connecting mechanism between a tanker and a yoke should be free rather than be fixed because the stress and the mooring lines' tensions are drastically decreased.