An effective demodulation of single closed interferogram was proposed. The initial phase of the closed interferogram was estimated by using of Fourier transform, band pass filtering and inverse Fourier transform. Then the phase was obtained accurately by image segmentation, sign correction and Zernike polynomial fitting. Image segmentation and sign correction solve the problem of phase ambiguity, and Zernike polynomial fitting reduces the local phase error near the segmentation line . The qualitative relation between the residual phase error and the parameters of interferogram were analyzed by simulation, such as tilt, defocus and noise. Experimental result shows that the extracted phase by the proposed method is in good agreement with that tested by the ZYGO phase-shift interferometer and the residual phase error is less than one tenth of the reference phase. Numerical simulation demonstrates the effectiveness of this method.