The identification of time-variant modal parameters by modified Littlewood－Paley (L-P) wavelet transformation is studied. The responses of structure subjected to dynamic load are transformed by modified L-P wavelet. The bands with significant energy content are the bands where the natural frequencies are located. The changes of the natural frequencies with time are determined by the wavelet coefficients graph. The mode shape is estimated by the ratio of the wavelet coefficients of two considered degrees of freedom at an instant time, corresponding to the band of freqency in which the natural frequency lies. The damage of shear frame structure is detected and the time when the damage occurs is estimated by the change of the frequency identified by modified L-P wavelet. The damage location is detected by the change of the slope of mode shape from the undamaged structure to the damaged one. By this method stucture damage is detected without excitation information, even when there is noise in recorded responses.