The residual direct current (RDC) almost always brings serious image sticking (IS) problems in LCDs and is mainly related to the liquid crystal (LC) and photoaligned polyimide. In this paper, we propose a novel method, to the best of our knowledge, to evaluate the RDC of the FFS-LCDs through an optical measurement system. By this means, the accumulation and release of the ions can be seen distinctly through the transmittance-time curves with the voltage regulation. Hence, it is helpful to compare and analyze the RDC problem of different displays. Moreover, this method possesses the advantage of high efficiency and simplicity in order to benefit the material design in photoaligned polyimide or the LC.