The epitaxial growth of metastable fcc Mn thin films on GaAs(001) surface has been achieved at a substrate temperature of 400K. The development of the fcc Mn thin films as a function of coverage is studied by photoemission with synchrotron radiation. The electron density of states below the Fermi edge of the fcc Mn phase is measured A significant difference of the electronic structures is observed between the metastable fcc Mn phase and the thermodynamically stable α-Mn phase. Possible mechanisms are proposed to interprete the experimental result.