Abstract: Q345 steel plates with thickness of 60 mm were joined by electron beam welding and the microstructure and microhardness distribution in the weld zone were studied. The results show that the 60 mm thick Q345 steel plates were welded with full penetration by the single pass electron beam welding. The welded joints in good shapes and without obvious defects such as pores and cracks were obtained. The weld depth-to-width ratio was as high as 15:1. The microstructure at the top of weld zone consisted of proeutectoid ferrite, side-plate ferrite and acicular ferrite. Along the depth of the weld, the amount of ferrite decreased while that of the lath martensite and acicular ferrite increased, the spacing between acicular ferrites became smaller and the grain size was reduced. From top layer to root layer of the weld, the hardness in the weld center showed a fluctuant increasing trend. The microhardness of the weld zone was higher than that of base material and heat affected zone. The increasing rate of the hardness in the weld zone increased obviously along the depth.