Type X carbonyl iron powder mixed with different proportions of paraffin were used for testing permeabilities and permittivities of these materials to study interference phenomenon of electromagnetic wave in radar absorbing coatings （RACs）. The formula of reflection coefficient of type X RACs to plane electromagnetic wave with the thickness of quarter-wavelength was solved by the method of multiple reflections. And the formula of reflection coefficient of type X RAC to plane electromagnetic wave of frequency at 9.5 GHz with different thickness was also solved in which the first six terms were chosen for calculating reflection coefficient Γ_a and reflectivity R_a. The results show that interference phenomenon is caused by the multiple reflections of wave. The value of Γ_a and the value of reflection coefficient Γ calculated by method of transmission line are close to each other which shows Γ_a and R_a can approximately represent Γ and reflectivity R of type X RAC separately. If the thickness of type X RAC is smaller than 1.9 mm or in the range from 3.2 to 6.6 mm， the second reflected wave will enhance the first reflected wave and increase the reflectivity which indicates the optimum thickness of type X RAC is in the area from 1.9 to 3.2 mm.