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The fatigue contrast tests of unload longitudinal direction cer joints as original welded and treated by ultrasonic peening of Q235B in various stress ratio are directed. The improvements of fatigue performance of unload longitudinal direction cer joints resulted by ultrasonic peening are studied. The effect patt of stress ratio on fatigue performance of welded joints that are treated by ultrasonic peening is studied. As tests results indicate that: ① In the condition of stress ratio R= -1, the fatigue strength of specimen treated by ultrasonic peening is increased by 165% of that of the original welded specimen. And the fatigue life of specimen treated by ultrasonic peening is as much as 75～210 times of that of the latter. When R=0.1, the fatigue strength is increased by 87% and the fatigue life is extended by 21～29 times. When R= -0.5, the fatigue strength is increased by 123% and the fatigue life is extended by 42～59 times. When R=0.45, the fatigue strength is increased by 51% and the fatigue life is extended by 3～14 times. ② If the welded joints are treated by ultrasonic peening, the fatigue strength is no longer independent on the applied mean stress. The more the stress ratio R, the less the fatigue stress range which can be sustained by the joints is. ③ Whether the high value residual stress is in the joints or not, the dead load portion of the applied load must be considered in the design of the joints which should be treated by ultrasonic peening.