Thermodynamics of hydrogen absorption and phase transformations in Ti6Al4V alloy were investigated by pressure-composition (P-C) isotherm measurement at hydrogenation temperatures in the range of 823~1023 K. Results show that the hydrogen pressure is increased with increasing the hydrogen content when Ti6Al4V alloy is hydrogenated at different temperatures. Only one sloped pressure plateau occurs in each P-C isotherm during the hydrogenation treatment because of the existence of original β phase in Ti6Al4V alloy. According to Vant's Hoff law, the values of enthalpy and entropy of the pressure plateau region are -50.7±0.26 kJ/mol and -138.4±0.69 J·K^-1·mol^-1, respectively. The Sieverts constant increases firstly and then decreases gradually with increasing the hydrogenation temperature. The phase composition and phase transformation of Ti6Al4V alloy during the hydrogenation treatment were analyzed.