On April 14th, 2010, a large earthquake (Ms=7.1) shocked the Yushu county in the Qinghai Province at western China. It caused severe damages and fatalities. The purpose of this paper is to estimate the building damages caused by the earthquake based on pre and post earthquake ALOS SAR data. The distribution of the buildings in the Yushu city was extracted from the intensity images of the SAR data. Phase coherence between two scenes of SAR data were calculated to assess the building damages. The results show that the buildings close to the surface rupture caused by the earthquake are broken severely. The further to the surface rupture, the less damages of the buildings. It indicates that avoiding the potential rupture zone in the urban planning is necessary to mitigate the earthquake hazards.