We investigate theoretically Cu 2p-XES for La2CuO4 and CuO on the basis of the formula of the second order optical process with the impurity Anderson model. According to Cu 2p-XES experiments so far made, a shoulder structure appears on the high energy side of the main peak in La2CuO4 and also in high-T(c) superconductors, but it is absent in CuO. It is shown that the appearance of the shoulder structure originates from the stronger hybridization for the a1g symmetry states in La2CuO4 than in CuO due to the existence of the apical oxygen.