When an MHD wave impinges on a fast shock from its upstream side, in general it may excite six diverging waves on the downstream side when the angle of incidence, the angle between the normal direction of the incident wave and the shock normal, is less than a critical angle. The boundary conditions across the shock surface determine the perturbations excited on the downstream side of the shock. We introduce a wave function approach to study the interaction of an MHD wave with an fast shock. When the angles of incidence are greater than certain critical values, less than six diverging waves can be excited; the flow becomes unstable on the downstream side of the shock. The existence of critical angles of incidence infers that the conclusion obtained on the stability of fast MHD shock with respect to normal incidence of MHD disturbances does not hold true for oblique incidence of disturbances at large angles of incidence. We investigate the critical angles of incidence, which are calculated as functions of the mode of the incident wave and three upstream conditions of the shock: the fast Mach number, the plasma &bgr; value and the shock angle. Our numerical results show that the critical angles of incidence are of the order of 60¿. ¿ American Geophysical Union 1987