The difference synthetic aperture radar interferometer (D-InSAR) is developed based on the synthetic aperture radar (InSAR) technique. It can be used to detect the earth surface deformation, fire hazard, earthquake and etc.. The imaging principle and data process procedure are introduced. The difference interfere process of the SAR data from Bam in Iran where earthquake happened on Dec. 26th, 2003 is implemented. The distinguish between the interferegrams before and after the earthquake is discussed. The phase image reflected the earth surface deformation arise from the earthquake is obtained. Thus the damaged area and the earthquake fracture zone position can be determined. Results show that the D-InSAR technique can be used to study the parameters of the earthquake and supply a new tool to help people to comprehensive the earthquake further.