The carbonization conditions (acetylene pressure and heating rate) to obtain close carbonized layer covered on Si(001) substrate without thermal pits is studied. Subsequent hetero-epitaxial 3C-SiC with smooth surface have been grown by low-pressure CVD. Single-crystalline carbonized layers could be grown at 1050°C by using suitable carbonization processes. The surfaces of Si were covered with single-crystalline 3C-SiC layers at an early stage of carbonization, preventing out-diffusion of Si atoms from Si substrates. 3C-SiC epi-film have RMS = 0.4nm but no single domain. The protrusion density of the film was an order of 1000 cm-2.