X-ray reflectivity has been used to characterize the interfacial structure of as-grown SiO2 layers deposited on Si(lll) substrates by rf magnetron sputtering under different substrate temperatures. Results indicate that there exists a higher-density interfacial layer between crystalline Si and the amorphous SiO2 overlayer. As the substrate temperature was increased from 200 to 620 degrees C, the density of the interfacial layer increased from 2.37 to 2.67 g/cm(3), and the thickness of the interfacial layer also increased slightly.