This study introduces an aeroelastic model for the linear/non-linear analysis of thick fins in supersonic or hypersonic regimes. In the first step a linear aeroelastic model for the analysis of thick fins is developed. To this aim, a thick fin with two degrees of freedom (2 DOF) as well as an elastic double-wedged fin in supersonic/hypersonic flight regimes are considered. An unsteady aerodynamic model is developed based on the shock/expansion theory by local aspplication of the piston theory over the flat surfaces of the fin. The structural model is also obtained based on the Lagrangian approach. Employing such developed model, the effects of initial angle of attack, thickness and some other geometrical parameters on the aeroelastic stability boundaries and unsteady aerodynamic loads are studied. In the next step, a more sophisticated model describing the non-linear aeroelastic behavior of fins with 3 DOF and free-plays in flapping, plunging and pitching motions is investigated. To this aim, governing equation are obtained by a modification of the linear model and effects of several geometrical parameters (e.g. thickness, initial angle of attack, hinge frictional torque etc.) on the aeroelastic behavior of fins are assessed1.