The adsorbed state of thiophene on Si(100)-(2X1) surface at 300 K has been investigated using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and ultraviolet photoelectron spectroscopy (UPS). (2X1) LEED pattern at 300 K is sustained after the saturated exposure of thiophene, and the saturation coverage is estimated to be similar to 0.6 by AES, suggesting that thiophene molecule is chemisorbed molecularly on the Si(100) surface most likely by sigma bonds between C and Si atoms. UPS spectrum for the chemisorbed thiophene shows not only the pi orbital shift but also the sigma orbital shift. Semiempirical PM3 calculations based on the cluster model propose that the thiophene molecule adsorbs on the Si(100)-(2X1) surface by forming di-sigma bonds between C atoms of thiophene and Si atoms of the surface.