The in situ annealing conditions of pulsed laser deposited MgB2 films were studied. The precursor films were deposited at 250 C from a stoichiometric MgB2 target in a 120mTorr Ar atmosphere. The films were then in situ annealed at a temperature from 450 C to 800 C and an annealing time from 1 minute to 10 minutes. We found that the superconducting properties depend in a crucial way on the annealing conditions: temperature, heating rate and time. The best film with a thickness of ~600nm was

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... ned under the following annealing conditions: Tanneal=680-690 C, tanneal=1 min, heating rate= 38 C/min. The Tc onset of the film is 28K with a transition width of ~10K. The hysteresis loop of magnetic moment of the film indicates weak field dependence in high fields. Magneto-optical imaging of the film showed quite homogeneous magnetic flux penetration, indicating structural homogeneity. The films without annealing showed no superconductivity.