Abstract: Lu_xY_1-xAlO₃：Ce single crystal samples were prepared by the Czochralski method. By adopting micropolariscope, field emission stereoscan, electron probe and thermal stress analysis, the macroscopic defects in Lu_xY_1-xAlO₃：Ce single crystals such as split, iridium inclusion, gas hole, gas cavity, twin, cleavage were investigated. It is supposed that, in the crystals of different shapes, the direction of the composite force of thermal stress varies, thus resulting in the different orientations of cracked section. Factors including enhancing heat preservation, installing a suitable temperature field in accordance with crystal shapes may contribute to the acquisition of perfect Lu_xY_1-xAlO₃：Ce single crystals. High temperature melt erodes iridium crucibles, and the microcontent oxygen in protective atmosphere oxidizes iridium crucibles, thus resulting in the iridium inclusions in Lu_xY_1-xAlO₃：Cesingle crystals. Decreasing oxygen content in the furnace, pressure die casting iridium crucibles using iridium of high purity and other factors may help reduce iridium inclusions. With increasing melt temperature moderately to decrease its viscosity and air pressure in the furnace, gas inclusions decrease. The cell parameters a and b are very close in Lu_xY_1-xAlO₃：Ce single crystals with low Lutetium content, and the adjacent cell parameters interchange with the inducement of stress, thus leading to twining along <110> direction.

Key words: Lu_xY_1-xAlO₃：Ce crystals, defects, thermal stress