This paper presents a local piston theory for calculating supersonic unsteady aerodynamic loads due to structural motion or deformation. A steady flow solution is first obtained by Euler method. The piston theory is applied locally at each point on the airfoil surface on top of the mean steady flow field to obtain the unsteady pressure perturbations caused by the deviation of the airfoil surface from its mean location. Computations by this method are performed for a number of unsteady flows and flutter problem. The results are compared with those by the classical piston theory and fully unsteady Euler codes. Since the piston theory is used only locally, this method greatly reduces the limitations of the classical piston theory on flight Mach number, the shape of the airfoil, and angle of attack. Compared to the fully unsteady Euler method, the localpiston theory method is more efficient as only one steady-state solution is needed for the flutter computation.