Abstract: With the Three-Coordinate Measuring Machine,the dimensional instability of LY12CZ welded specimens under the condition of thermal cycles was investigated experimentally.The deformation mecha-nism was analyzed from the viewpoint of mechanics and microstructures.Experimental results show that the dimensions of LY12CE welded specimens will tend to stable after mine thermal cycles,and the reasons for in-stable deformation of welded specimens are tightly related with temperature,change of mierostructures and relief of residual welding stress.