Abstract: The P-C-T curves of LaNi_4.25Al_0.75 alloy absorbing protium, deuterium and tritium were investigated at constant temperature and volume in our metal hydride research system. According to the Van’t Hoff Equation, the thermodynamics parameters of the hydride phase were determined. Enthalpy change and entropy change for protium, deuterium and tritium were -44.5, -45.0, -47.1 kJ/mol and -118.0, -121.8, -127.5 J•mol^-1•K^-1. The experiments indicated that the isotope effect did not obviously at low temperature, however, as the temperature increasing to 100 ℃, there were obvious thermodynamics isotope effect with hydrogen absorption by LaNi_4.25Al_0.75 alloy. The order of equilibrium pressure absorbing gases was T₂>D₂>H₂. However, the enthalpy and entropy appeared to be in verse order, T₂>D₂>H₂. The results showed that hydrogen isotope effect with LaNiAl alloys were depended on temperature.