Abstract: Three-wave coupling process in parametric excitation is well studied under an isotropic, homogenous and collisionless ionospheric background. First discussed is the nonlinear coupling between ionospheric eigen oscillations with the presence of incident electromagnetic wave. Then, the dispersion relation of parametric excitation process in collisionless ionosphere is deduced, and two kinds of frequency conditions and wave-vector conditions are obtained for critical excitation of parametric insta bility. It is proved theoretically that the excitation of an electron Langmuir wave and an ion acoustic wave when parametric instabilities occur is the easiest tendency due to the minimum energy consumption.