Adsorption states of three types of C 6 cyclic hydrocarbons, i.e. cyclohexane, cyclohexene and 1,4-cyclohexadiene, on Si(001) 2x1 reconstructed surfaces are investigated by photoemission electron spectroscopy, high-resolution electron energy loss spectroscopy and temperature programmed desorption (TPD). We show that unsaturated hydrocarbons, cyclohexene and 1,4-cyclohexadiene, are chemisorbed at reconstructed Si dimers to form stable di-σ bonds, while saturated cyclohexane is weakly physisorbed on the surface. During annealing of the precovered surface, dehydrogenation of chemisorbed molecules into benzene was expected on the surface above 100 o C. Pentacene films were deposited on the Si(001) with 1,4-cyclohexadiene chemisorbed layer and that with SiO 2 oxide layer. Large grains reaching 1μm were obserbed in the film on the 1,4-cyclohexadiene-buffered surface in contrast with smaller grains grown on the SiO 2 surface.