The micro-hardness of welded joint was measured,and the microstructure and fracture morphology of welded joints had been observed by using metallographic and scanning electron microscope(SEM).The results show that Omega weld metal in stainless steel manual valve is coarse columnar crystal,and exists strong direction of grains(columnar and cellular crystal),and intergranular segregation,intergranular cracking in weld had formed along the grain boundary.The main reasons are that the liquid filling of the assembly gap region of Omega weld is not enough during process of cooling crystallization,so that shrinkage porosity produced.The welding solidification crack will be formed if the tensile strain or strain rate exceeds the critical plasticity and the critical strain rate of material.