As-grown superconducting MgB 2 thin films have been grown on SrTiO 3 (001), MgO(001), and Al 2 O 3 (0001) substrates by a molecular beam epitaxy (MBE) method with novel co-evaporation conditions of low deposition rate in ultra-high vacuum. The structural and physical properties of the films were studied by RHEED, XRD, electrical resistivity measurements, and SQUID magnetometer. The RHEED patterns indicate three-dimensional growth for MgB 2 . The highest T c determined by resistivity measurement was about 36K in these samples. And a clear Meissner effect below T c was observed using magnetic susceptibility measurement. We will discuss the influence of B buffer layer on the structural and physical properties