The microstructure in adiabatic shear band (ASB) of U-5.7Nb alloy deformed at high strain rate by a split Hopkinson pressure bar (SHPB) was investigated by means of OM, SEM, and TEM. The ASB with black etching color in U-5.7Nb has been observed, and its width was about 40 um. The evolution of microcracks in ASB led to the formation of the micro-feature which like equiaxed "broken grain" with the size of 2-5um in ASB. The nanometre-sized grain domains well below 5 nm are found in the core of ASB, and the orientations of single small grain domain differ completely, even within an area of 1 μm. Microhardness of the ASB is higher than that of the matrix, but elastic modulus of ASB is lower than that of the matrix. The different trends between microhardeness and elastic modulus are caused by the change of microstructure in ASB.