The evolution of dislocation microstructure in electron beam melted Ta-2.5W alloy was investigated.Long straight dislocations and dislocation loops were formed in Ta-2.5W alloy cold-rolled by 5%.A group of long,continuous extending planar boundaries(EPBs)were formed when the reduction reached 20%.The early stages of the EPBs were fragmented,diffused and curved,which were connected by non-crystallographic cells boundaries to maintain continuity.The straight segments of EPBs were usually developed on {110} planes,and inclined at about 25~35° to the rolling direction(RD).Two groups of EPBs were formed when the reduction was larger than 30%.The dislocations within EPBs tended to rearrange themselves with increasing strain in a sequence,from tangled dislocations,followed by parallel long straight screw dislocations and finally into dislocation nets.The regular dislocation nets were composed by 1/2<111> and [100] type dislocations.The EPBs were not strictly planar and appeared wavy,which can be attributed to the relaxation process of dislocations and the interaction of dislocations from different slip planes.