Based on the studies of on the many workers, the microstructures of the duplex-phase Ti-6Al-4V alloys are α’ or α” martenisite during the quenching from the β phase or the (α+β) phase regions. In addition, the α′ phase could be identified to belonging the H.C.P., F.C.C. or F.C.T. structure. Besides, α″ phase can be identified to belonging the H.C.P. or orthorhombic structure. Based on the present studies, being solution heated treatment in the single β region on the 1100 °C for various quenching rates, the microstructure of the Ti-6Al-4V alloy is essentially the martenite with the high-density dislocation, and the martensite could be identified to be two-type H.C.P precipitates formed by spinodal decomposition. When aged at 950°C in the single (α+β) region, the microstructure of the alloy is the mixture (α’+β+ω) phases. When aged at 850°C in the single (α+β) region below the martensite transition temperature, the microstructure of the alloy is the mixture (α+β+x) phases. In addition, the x phase would be belonging to the F.C.C. structure with the lattice parameter a=0.426 nm.