Motion control system using small fins is necessary for Resonance-Free SWATH (RFS) which has negative pitch restoring moment. Theoretical estimation method regarding Bode plot which determines the stability of PD control system and motion equation including PD control system is developed. In the theory, the time lag of fin lift generation and the interaction among fins and lower hulls of RFS are taken into account. Also, experiments using RFS model with movable fins are carried out to validate the theoretical estimation. As the result, the maximum stable gain constants for PD control of RFS motions are determined and motion responses of RFS are significantly reduced. Furthermore, non-linear VSS control system is introduced to make a good use of negative restoring moment of RFS. Sliding mode control and the effect of VSS system are discussed.