Abstract: This paper presents a case study of stability analysis in a huge underground powerhouse. Solid models of caverns with fractures are created using AutoCAD2002/2004 software with programmed modules for excavations/cutting, which specify arch crown, upstream side wall and downstream side wall as cross sectional planes. Based on these fractures, 21 blocks are chosen as deterministic blocks and their stabilities are then assessed. At last, assessment results are given for all deterministic blocks including following attributes: fractures jointed methods, sliding methods, area, depth, volume, safety factor, and location in caverns.