The surface electronic states of the clean (and GO-covered) Si(001) c(4x2) and c(8x8) surfaces at 90 K have been studied by using high resolution electron energy loss spectroscopy. On the clean c(8x8) surface, the loss peaks are observed at 0.3, 0.7 and 1.2 eV. The 1.2 eV loss is sensitive to the CO adsorption, whereas the 0.3 and 0.7 eV losses are not sensitive. These suggest that the c(8x8) structure contains the defects which are not understood by the dimer vacancy model.