Xenon spatial oscillation may give rise to operational difficulties in a nuclear power plant. In this study, in order to investigate xenon instability for a pressurized water reactor, the frequency-domain technique is adopted by using the generalized Nyquist criterion, which is more general and suitable for the multi-input /multi-output system. Also linearized modal fluxes are obtained by modal expansion. This model has been implemented to test the axial xenon stability of the KNU 7 unit with changes in plant operating parameters.