The properties and distribution of defects in Nd:YAG crystals are successfully obtained by means of X-ray transmission topography with AgKa radiation. It is shown that there are several kinds of defects, such as growth striations. precipitated particles. edge dislocations, screw dislocations and mixed dislocations formed by dislocation motions in 2Sd:YAG crystals grown by CZ and Temperature Gradient Techniques (TGT). Some experimental results are consistent with the results obtained by optical methods, however，X-ray method bad its special advantage in discriminating the properties of dislocations.