The interface formation of room temperature deporsited Mn on GaAs(100)(4×1) surface was studied by low energy electron diffraction (LEED), X-ray photoelectron spec-troscopy (XPS), electron energy loss spectroscopy (EELS) and ultra-violet photoelectron spectroscopy (UPS). It was found that above the Mn coverage of 0. 25 nm, the LEED pattern disappeared completely, which indicated that the Mn overlayer did not form single crystalline structure. The layer-by-layer growth mode at the initial stage was proved by the results of both LEED and EELS. By careful study of the intensities and line shapes of the Ga2p3/2 and As2p3/2 core level photoemission spectra, we observed that the exchange reaction between the deposited Mn and the substrate occurred at very early stage. The Ga atoms released by the exchange reaction distributed within a region about 3 nm above the original interface, while the As atoms which combined with Mn to form As-Mn compound segregated on the top of Mn overlayer. The layer-by-layer growth mode persisted until the latest stage of the deposition. Some of the above results were intuitively confirmed by UPS measurements.