Abstract: Based on the latest gravity and magnetic data, we inverted basement depth of north Xihu sag,deduced and explained its igneous rock and fault structure. Together with relevant geological and structural data, we also analyzed basement and structural characteristics of north Xihu sag in order to further under-stand basement characteristics of the sag. Study results show that faults in the study area are mainly of two kinds of striking directions:northeast and northwest. Faults with NE striking are regionally much older in ages,and with interveined by NW ones,they look more complicated in shape. Volcanic rocks are developed well in the Xihu sag, anti distributed along the large scale fault belts with a beaming shape in space. The distribution of volcanic rocks is clearly controlled by the faults, especially at the intersection area of the NE-and NW-striking faults. Tertiary basement structures in the north Xihu sag are mainly striking in NE direction, but significantly replaced by the NW striking faults,thus showing "zoning in E-W direction and blocking in S-N direction" macro-scale shaping characteristics. This basement undulated greatly with more than 5 000 m depth below the seafloor, and was generally higher in south than in north. From east to west, there are developed 5 substructrure units in the basement, namely, Baojiao slope, Santan deep-depression,east Zhejiang anticline belt, Baidi depression and Tianping faulting belt.