Surface-strengthening treatment was performed on the welded joints and base material specimens of AISI304 austenitic stainless steel using ultrasonic peening treatment. Stress-corrosion test in a 42% boiling magnesium chloride solution were conducted with treated and untreated specimens. Four-point loaded specimens were employed in the stress-corrosion test. Residual stress in the surface layer of the specimens was analyzed with an x-ray stress analyzer before the stress corrosion test. The microstructure of the specimens was observed by a scanning electron microscope after the stress corrosion test. The state of stress in the surface layer of the specimens affected by the ultrasonic peening treatment was analyzed. The stress corrosion cracking behavior was studied by observing the corrosion fractographs. The results showed that the stress corrosion resistance of the welded joint and base material specimens was remarkably improved because of the compressive residual stress employed by the ultrasonic peening treatment. Fracture modes were of the same kinds in the treated and untreated specimens.