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Although the as-grown method is becoming the main current for fabrication of MgB2 thin films, the in-situ annealing method is still indispensable and effective for obtaining thin films with a high zero resistivity temperature and current density. We have investigated the effects of in-situ annealing on the superconducting properties of MgB2 thin films. MgB2 thin films were fabricated by electron beam epitaxy on C-plane sapphire (Al2O3) substrate. Thin films were produced by simultaneously evaporating B and Mg metal in a high vacuum of 3×10-7 Pa. Substrate temperature was 265°C. The zero resistivity temperature of the as-grown thin films was 31 K. Electron beam deposition was followed by in-situ annealing in a high vacuum. In the optimal annealing process (a thin film is heated at 600°C for one hour) we have consistently obtained thin films with a zero resistivity temperature of 35 K. In contrast, the critical current density of MgB2 thin films was decreased by annealing. This suggests that the crystal grain boundaries in the MgB2 thin films are working as a flux pinning site, and crystal growth from annealing brought about the reduction of current density.