The principle of differential interferometric （D-In） technique, the method for selecting proper D-In data pairs, and the flowchart of 3-pass D-In data processing are introduced firstly. Then using the differential interferometric on synthetic aperture radar（D-InSAR） technique and the ENVISAT ASAR data, the monitering to co-seismic deformation field in the Barn Mw6.5 earthquake in Iran, on December 26, 2003, is taken as application. As the result, the butterfly shap co-seismic deformation field of the Barn earthquake is gotten successfully, the isoline map of the defermation is generated, and the position, distribution and area of seriously destruction region are determined according to the coherence map. The test proves that D-InSAR technique is a powerful tool to measure surface deformation and to study earth- quake.