A hot-filament chemical-vapor deposition (HFCVD) technique was presented for the direct in situ synthesis of MgB2 thin films. MgB2 superconducting films were in situ prepared on (0 0 0 1) sapphire substrates using this technique. The phase constituent, surface morphology, and the critical superconducting transition temperature of films were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and DC SQUID magnetometer, respectively. Experimental results show that the formation of MgO impurities can be suppressed in MgB2 films grown by this technique. As the substrate temperature increases from 400 to 600 degrees C, both the crystallinity and densification of films increases, and superconducting critical transition temperatures of films increase from 18 to 36 K. A phase-pure MgB2, film can be obtained by depositing at 600 degrees C for 30 min. (c) 2006 Elsevier B.V. All rights reserved.