Hel ultraviolet photoemission studies of CO coadsorption with different K coverages on W (100) surface show that the 1π/5σ level of α-CO shifts from 8.6eV to 9.3eV relative to the Fermi level while the K coverages increase, this indicates the increase of electron backdonation into 2π* of α-CO. Two independent peaks are found around 5.5 eV (Characteristic of β-Co), one is at 5.2eV, another is at 6.0eV. The peak at 5.2eV grows with increasing K coverages, its energy position is the same as that of O coadsorption with K on W(100) surface, this suggests that K forms bond to O of β-CO.