Abstract: We introduce COSMO-SkyMed Earth observation system,and explore the relationship between coherence and high resolution SAR and Tandem constellation.A DEM of Mt.Qilian of China is generated by InSAR with COSMO-SkyMed Tandem data.We propose a method based on lowpass filtering on differential interferometric phase to estimate height error induced by atmosphere and orbit error,and analyze decorrelation factors.Compared with ASTER GDEM,the COSMO-SkyMed DEM shows more details.The experimental results demonstrate the potential of COSMO-SkyMed's application in InSAR DEM generation.