A method for non destructively measuring the complex permittivity of solid dielectric on a microwave branch coupler has been described in this paper. The principle of measuring the dielectric constant by using 3 dB branch coupler was illustrated. The device consists of a planar hybrid junction with the direct and coupled ports terminated by two identical open ended transmission lines;one being loaded with the material under test (MUT).The complex permittivity of the MUT was evaluated by simply measuring the amplitudes of the reflected and the transmitted signals of the measuring device.The complex permittivity of the solid dielectric such as Teflon and Polypopylene has been measured at 2.45 GHz by using the proposed method.The results are agreed with that reported on the literatures.