It is important to understand the microstructural evolution during water quenching after different processing conditions in order to obtain optimum mechanical properties of Ti-6Al-4V alloy. The thermal simulation of Ti-6Al-4V alloy was conducted using a thermomechanical simulator. The results have shown that a fully martensitic microstructure was obtained after solution treatment. Water-quenching after solution resulted in hardness increase, and this was attributed to the presence of α′ and α″. The lamellar structure formed when the specimen was water-quenched after isothermal holding at 850℃, and the hardness further increased. Colony appeared after deformation at 850℃ subsequent water quenching, and the hardness reached the maximum value (Hv584). This was attributed to the grain boundaries strengthening. The microstructure composed of α phase, lamellar colonies α/β and colony when the specimen was isothermal held at 600℃ after deformation at 850℃. Cooling and isothermal holding after deformation resulted in an increase of the width of α phase and colony. It offered a range of 324-706 nm for equiaxed grains. This resulted in the hardness decrease.