Abstract: The effect of ultrasonic impact treatment (UIT) on the stress corrosion cracking (SCC) behavior of 304L/ER316L stainless steel welded part in boiling magnesium chloride solution was investigated by using U-bend specimens and double-beam bend specimens . The results showed that UIT caused a compressive stress deformation layer with thickness of about 1 mm on surface of the welded part, the effect of UIT on SCC was related to specimen type. There was little difference in the appearence time of U-bend specimen SCC cracks between the base metal (BM) area in high deformed and the welded parts before and after UIT, that is, UIT did not significantly improve the SCC resistance of welded part in such stress-strain condition. At stress levels close to and exceeding the yield strength, both groups of double-beam specimens showed the same SCC susceptibility ranking 304L/ER316L＞304L-BM＞304L/ER316L-UIT, which indicating that UIT significantly improved the SCC resistance of welded parts under the two stress. The crack propagation modes of all specimens were mixed intergranular and transgranular SCC fracture type. The order of SCC sensitivity in welded parts was weld zone