Abstract: Thermal and "Speckle" noise is an obstacle to generate digital elevation model(DEM) from the interferogram by 2-D unwrapping of Interferometric SAR. The boxing and median filters effects on reducing the noise are investigated in detail in this paper. Especially for the single complex images, it is more convenience to process the images by low-pass filters in image domain than in the frequency domain. And the window size effects on the processing are also discussed in detail. In order to process the noise by low pass filter reasonably, the flat earth removal is introduced to eliminate the frequency shift effect arisen from the geometry of the interferometric SAR. At the same time, the interferogram after earth flat removal can show the earth topography directly. The raw data show the validation of the algorithms in the processing.