Abstract: Low cycle fatigue tests had been carried out to investigate the cyclic behaviors of welded butt joint of structural steel. Cyclic stressstrain curves of weld metal, base metal and welded joint were obtained respectively by means of incremental step method in various strain levers with single sample method. The different cyclic stress-strain behaviors of three kinds of specimen were compared and analyzed. For butt joint specimen, the weld metal, heat affection zone and base metal, are loaded cyclically. It is proposed that the heterogeneity of mechanical property resulted from the heterogeneous microstructure of welded joint causes the different cyclic stress-strain properties from the other two kinds of specimen. The heterogeneity affecting fatigue behaviors of welded joint should be conceded during the design of boiler and pressure vessel structures.