The microstructural evolution during the equal channel angular pressing of Ti-6Al-4V alloy was investigated using the transmission electron microscopy (TEM). ECA pressing was carried out isothermally with route C at $600^{circ}C$ for two types of initial microstructure, i.e., equiaxed and Widmanstatten microstructures. At an initial stage of ECA pressing, the equiaxed microstructure showed more uniform flow than the Widmanstatten microstructure. However, both microstructures were significantly refined revealing nearly equiaxed grains of 0.3$mu extrm{m}$ in diameter with high angle grain boundaries after 4 passes of ECA pressing. These ultrafine gains were found to be stable with little grain growth, when annealed up to $600^{circ}C$ for 1hr.