In this work, the modeling of a Power Take- Off (PTO) unit for a point absorber wave energy converter is described. The PTO influences the energy conversion performance by its efficiency and by the damping force exerted, which affects the motion of the body. The state space model presented gives a description of the damping force and of the internal dynamics of the PTO. The aim of this work is to develop a model for the PTO as a part of a complete wave-to-wire model of a wave energy converter as in Figure 1, used for the design control techniques. Figure 1: Wave-to-wire model structure A bondgraph is employed to model the physical system that provides transparent and methodical means of formulating state space equations and of visualizing energy transfer throughout the system. Bondgraphs have already been shown to be a very useful tool for the modeling of PTO for wave energy converters (2). The dynamic of the mathematical model is then analyzed respect to the variation of parameters; in particular, the non-linear system obtained is linearized and its eigenvalues are calculated as function of the accumulator size and pre-charge pressure