Abstract: Actual values of shear strength parameters are important to rock mass as bedrock foundation of hydropower station dam.This paper presents large shear tests to measure the shear strength parameter values of the bedding planes sandstone in laboratory.The sandstone is excavated from the bedrock of a hydropower station dam.By means of the large shear tests,the characters of shear strength about sandstone structural planes are examined.Therefore good results are obtained.The tests show that the characters of the shear deformation curves are strain hardening and the displacements are up to 8.9 mm～44.5 mm before reaching to the peak points of shear strength.At the same time,the basic friction angle of the bedding plane is 31.4 .The average undulant angle of the bedding is 6.3.