The estimation of transfer function and impulse response of a system by the cross spectraltechnique is very popular and most of the FFT analyzer is equipped with that function. Inthe cross spectral technique, the impulse response changes its form by the use of timewindow. But the deformation of the impulse response in the process of estimation is not yetwell known. This paper first theoretically describes why and how the impulse responsechanges its form. According to the theoretical derivation, the deformation of the impulseresponse is illustrated for several commonly used time windows and the results are verified by the computer simulation.