Abstract: 50 mm thick 304 stainless steel plates were welded using K110 electron beam welding (EBW) machine with high voltage vacuum. Microstructure and hardness along the weld center line were tested with optical microscope, scanning electron microscope and microhardness tester. The results show that complete defect free butt joints could be obtained by one pass with electron beam welding. The weld depth-to-width ratio reached as high as 18:1. From the top to the depth of 39 mm, ferrite distributed in austenite grain boundaries or between dendrites with net, lathy/skeletal and equiaxed dendrite morphologies. At the weld bottom, the cellular metastable austenite would precipitate directly as primary phase from the melt, replacing stable ferrite. The weld microstructure consisted of austenite and ferrite. Solidification mode of the weld transformed from FA to AF mode along the weld center line because of the increasing cooling rate. The grain size reduced and the hardness in weld center fluctuated but grew along the weld depth direction.