Abstract: The welded joints obtained on the condition of different electron beam (EB) heat input mode were studied by means of optical microscope, scanning electron microscope, transmission electron microscope and microhardness measurement. The results indicate that the heat input energy and distribution pattern have an important effect on the size of the weld grain. The weld microstructure was uniform with grain of size 350 Ixm when midium heat input E=48.0 kJ/m was utilized. With the increment of heat input, coarse needle-like α+β phase and martensite of weld metal were transfered from unidirectional long needle to mahi-direcitonal short needle morphology and dislocation density was enhanced although less than that of the base metal. Microhardness distribution in weld zone with different EB heat input was greatly different which was possibly attributed to different phase volume and distribution formed metastale. When weld heat input was E=48.0 kJ/m, the joint microhardness distribution was uniform (404-480HV).