Abstract: Q345 steel welded joints were tested to failure under low cycle fatigue(LCF), high cycle fatigue(HCF) and combined cycle fatigue(CCF) using an apparatus that is capable of providing interactive LCF/HCF loading.The stress ratio R is 0.5 and the frequency of HCF is about 19 kHz.The results indicate that not only high frequency minor cycles superimposed on low frequency major cycles, but also low frequency minor cycles superimposed on high frequency major cycles can do remarkable damage to fatigue performance of welded joints.The CCF strength is characterized by amplitude envelope.While CCF fatigue life is characterized by LCF life, adverse effect of HCF component is underestimated.If CCF fatigue life is characterized by HCF life, adverse effect of LCF component is overrated.