From the physical optics assumption and Leontovich Impedance Boundary Condition （IBC）

we build up the bistatic formulae of the electromagnetic field scattered from three dimensional conduct object coated with radar absorb materials（RAM） with an arbitrary smooth convex shape.This problem is formulated in terms of Fresnel reflect coefficients and IBC. The interrelation of twoincident fields between the coated surface and the conducting surface of the same configuration

as well as the coated surface electromagnetic current ratio coefficients are also derived. At last

the computational method of bistatic scattering matrix and bistatic cross section from electrically large coated bodies is given for non-edge structures.