Abstract: This paper presents the results of an investigation of the influence of hydrogen treatment （hydrogenation + eutectoid tran-sformation + dehydroge-nation） on microstructure of Ti-6Al-4V cast alloy. It was shown that the f. c. c. TiH2 plate precipitated in hydrogenated specimens. As the hydrogen content in the specimens increased, the TiH2 phase spheroidized. During eutec-toid transformation, the grain boundary a phase disappeared and the a + TiH2 eutectoid microstructure with few (3 phase Formed.The dehydrogenation of hydrogenated specimens was conducted in vacuum under elevated temperature. The coarse Widmanstatten microstructure of cast titanium alloy, was broken up into very fine α+β equiaxed micro-structure by hydrogen treatment, which provides an effective way to improve the microstructure of Ti-6Al-4V cast alloy.