Abstract: The“water line”in nephrite from Qinghai Province is one of its most important characteristics.Analyzing the structure characteristics of“water line”in nephrite from Qinghai Province is very important for further research on the geological origin of“water line”and identification of the origin of nephrite.The structure characteristics and mineral composition of“water line”in nephrite from Qinghai Province are systematically analyzed by using the polarizing microscope，ESEM and EPMA.The results show that the mineral composition in the“water line”is tremolite.The“water line”in nephrite from Qinghai Province can be classified into the following three categories according to the size and order degree of arrangement of tremolite particles in the“water line”and in the subject mineral：the tremolite particles in the“water line”is bigger than those in the subject mineral，with a length of about 25μm and a width of about 4μm，and their arrangement is in a disorder；the tremolite particles in the“water line”are almost parallel to each other，with a length of 10－20μm and a width of about 2μm，meeting the boundary at an angle of nearly 90°；the tremolite particles in the“water line”are small，with a length of about 10μm and a width of 0.5－1.0 μm，and the boundary between the“water line”part and the subject mineral is very clear.ESEM results show that the tremolite particles，in a long fibrous form，are parallel to each other，in which there are a small amount of needle－like and granular tremolite particles.The orientation of the“water line”part is better than that of the subject mineral.