The elliptical reflection method was used to measure the electro-optic coefficient of doped α′-cyano-4′-nitro-4-N,N-dimethylstibene (CNDS)/PMMA films. The decay of electro-optic signal of CNDS/PMMA films with an coated Al electrode was measured in situ. A slow relaxation process was observed in the first few hours of the decay. Possible reasons were discussed.