As-grown Mg B2 thin films on Si substrates with high Jc under magnetic fields were prepared by electron-beam evaporation. The value of Jc has been enhanced by the deposition of Mg B2 thin film in an O2 atmosphere. The Mg B2 thin film deposited in the O2 atmosphere (O2 -doped film) has exhibited considerably higher Jc in magnetic fields among Mg B2 thin films reported before. It has been found that the high Jc of the O2 -doped film is attributable to the flux pinning with grain boundaries strengthened by an introduction of MgO along grain boundaries. In a high magnetic field, a peculiar behavior of E-J characteristics where E-J curves vary in two stages was observed. This behavior also originates from the flux pinning with strengthened grain boundaries.
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