The first-and second-order monostatic cross sections of swell contaminated seas for high-frequency ground wave pulsed radar operation are derived based on the fundamental electric field equations. The ocean surface, a mixture of both swell and wind wave components, is first represented with a Fourier series. The corresponding equations of the received electric field are then obtained. Fourier transformation of the autocorrelations of the electric field gives the Doppler power spectral density, which ultimately leads to the radar cross sections. Simulated results are also provided and are observed to be highly consistent with real Doppler spectra.