In this paper, the microwave (5GHz) and terahertz (1THz) EM (electromagnetic) wave scattering from sea surface covered with oil are studied in theory. The sea surface covered with oil can be regarded as a two-layered structure, the upper layer is oil and the lower layer is seawater. For microwave scattering, it is known that the oil covered on sea surface mainly influence the sea surface from two aspects: the damping effect for small scale wave and the reduction for friction velocity. In this work, the modification for Fresnel scattering coefficient due to the presentation of oil film is also taken into account. The numerical simulations show that the NRCS (normalized radar cross section) of microwave is not sensitive to the thickness of oil film. On the contrary, the NRCS of terahertz wave is more sensitive to the thickness of oil film, which provides us more information to probe oil slicks on sea surface with terahertz EM waves.