Abstract: By using Mao-Bell type diamond anvil cell (DAC) and an in situ high pressure powder X-ray diffraction method, the pressure induced phase transition and the isothermal compression of Fe62Ni27Mn11 (wt%) alloy were investigated at room temperature and high pressure up to 43.2 GPa. The crystal structure of this alloy is the fcc structure below 19.4 GPa. The pressure induced fcchcp phase transition occurs at 19.4 GPa, and then the fcc phase and the hcp phase coexisted up to 43.2 GPa with the same molar volume. The ratio of the lattice parameters of the hcp phase, c/a, keeps essentially a constant value of 1.6300.006 in the present high pressure range of the experiments. In the course of unloading, the hcp phase will remain its existence down to 5.8 GPa, and then transforms completely to the fcc structure as it is release to atmospheric pressure. The Murnaghan isothermal equation of state for this alloy was fitted with the parameters B0=(16612) GPa, B0'=5.20.5 by using the least-square method. A model for this pressure induced fcchcp phase transition is presented and the reason why it has so wide a two-phase-coexistence pressure region is also discussed.