Abstract: The hybrid vector finite element method and fast multipole method(FEM-FMM) was used to calculate RCS of 3-D complex targets coated with various radar absorbing materials(RAM). For a cone as an example, this paper analyzed the impact of four kinds of canonical RAM including lossy isotropic, positive uniaxial anisotropic, negative uniaxial anisotropic and nanometer absorbing materials on electromagnetic scattering characteristics of the objects. For the first time , the effect of RCS caused by different coating RAM thickness was discussed and optimal RAM with its corresponding coating thickness were obtained to attain best stealth purpose. Numerical results showed that the back RCS can be reduced about 16.6dB compared with the uncoated objects by using optical coating RAM.