This letter presents a polarimetric interferometer (PI) that can measure the ellipsometric parameter \theta with an accuracy of 0.01o leading to a potential accuracy of 17 pm. The PI is constructed and compared with a commercial heterodyne interferometer (HI). Given its low nonlinearity, the PI is used to measure the residual nonlinearity of a heterodyne interferometric displacement system. A rotating half-wave plate is used to compensate for a part of the nonlinearity error caused by the misalignment of the axis between input polarizing states and beamsplitter.