The performance of high frequency surface wave radar (HFSWR) may be significantly impacted by ionospheric clutter. A theoretical model of the ionospheric clutter for the case of vertical reflection is presented in this paper. The equation of the received electric field after vertical reflection by the ionosphere is first derived for a pulsed dipole source. Then the spectral density model of ionospheric irregularities is incorporated into the ionospheric clutter power model. The normalized ionospheric clutter power is simulated with various horizontal ionospheric plasma drift velocities.