Earthquake is an uncertain and random phenomenon. Therefore, it is so much important to find an excitation which can create the highest crisis in a structure under some specific constraints of the earthquake such as earthquake intensity and amplitude limit of power spectral density (PSD). The critical excitation proposed here, in this paper, is introduced in the frequency domain and the objective function is the maximization of the mean square of story drift. The proposed critical excitation has the capability to cover the frequency range of power spectral density function and to have maximum responses. For this purpose, effort is made to adapt a linear combination of F(ω) and Kanai-Tajimi equation as the critical excitation, where F(ω) represents critical response of the structure and Kanai-Tajimi equation is PSD of the past natural earthquakes. These excitations are referred to as continuous critical excitation. Also, single degree of freedom structures are investigated in the stationary sate. Finally, the proposed method is compared with the other's method of finding critical excitation.