This paper presents numerical simulations and analyses of EM (electromagnetic) scattering from oil-free and oil-covered sea surface. First, the influences caused by slicks on clean sea are studied and analyzed with the action balance function. Slicks on sea surface make significant impacts on the wind input, the nonlinear wave-wave interactions and the viscous dissipation. A damping model based on the action balance equation is introduced. And then, simulations are made by assuming the surface height spectra proposed by Elfouhaily et al. and Hwang, respectively. The two scale model (TSM) is used to calculate the normalized radar cross sections (NRCS) of oil-free and oil- covered sea surfaces. Additionally, an UAVSAR image which was collected during the Deep Water Horizon oil spill accident occurred in the Gulf of Mexico is served as a reference. The numerical comparisons between simulated results and measured data have shown that, for clean sea surface, the VV polarized NRCS simulated with Elfouhaily spectrum agree well with UAVSAR data, the HV polarized NRCS simulated with Hwang spectrum agree well with UAVSAR data. For polluted sea surface, the VV polarized NRCS simulated with Elfouhaily spectrum matches well with measured data. Overall, numerical simulations with Elfouhaily spectrum seem better than Hwang spectrum in our simulations.