A new method is developed for determining of retardations and azimuth angles in two-ply retarder. This method is based on Senarmont's compensator. But linearly, circularly and elliptic polarized light and two monochromatic lights are used in this method. It is discussed in detail by the Stokes'parameter how a state of incident polarized light is transformed by the two-ply retarder on the Poincare sphere. Each azimuth angle and retardation can be measured without separating the test piece into each retarder. Good agreements are obtained between experimental and theoretical values.