Hot deformation behavior and microstructural evolution of Ti-6A1-4V alloy in case of α' martensite starting microstructure were examined by comparing the case in equilibrium (α+β) starting microstructure. Compression test was carried out at 1173 K and 1273 K at strain rates of 0.001s -1 and 1s -1. In compression test in temperature region of (α+β), equiaxed grained structure is exhibited for both starting microstructures of α' martensite and equilibrium (α+β) after compression test. Then, distortion-less ultrafine grained microstructure is obtained in case of α' martensite starting microstructure after hot compression at condition of 1173 K-1s -1. While, the lamellar microstructure consisting of α phase and β phase is obtained after compression at 1273 K, which is above the β-transus of 1268 K, for both the cases of α' starting microstructure and as-received (α+β) starting microstructure. These results indicate that hot deformation behavior in the cases of α' martensite- and (α+β) starting microstructure is different especially at the temperature (1173 K) which is below the β-transus.