Microstructure evolution and grain refinement of Ti-6A1-4V alloy after laser shock processing (LSP) are systematically investigated in this paper. Laser shock waves were induced by a Q-switched Nd:YAG laser system operated with a wave-length of 1064 nm and 10 ns pulse width. The microstructures of LSP samples were characterized by scanning electron microscopy (SEM) and transmission electron microscope (TEM). Present results indicate that the surface hardness of samples subjected to LSP impacts has significantly improved. Multidirectional twin intersections and dislocation movements lead to grain subdivision in alpha phase with ultra-high plastic deformation. High-density dislocations are found in beta phase. Multidirectional twin intersections and division of sub-grain boundaries play an important role in the grain refinement of Ti-6A1-4V alloy under LSP loading conditions. (C) 2015 Elsevier B.V. All rights reserved.