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Mg B 2 superconducting thin films on Si(111) and Al2O3(0001) substrates were prepared by high vacuum e-beam evaporation and two-step in situ annealing techniques. The precursor films [B(100Å)∕Mg(151Å)]6∕Al2O3 (or Si) were deposited at room temperature and 1×10−7mbar of background vacuum, then annealed in situ at 630°C for 30min in an argon atmosphere of 150Pa. The atomic force microscopy image showed that the films were uniform with grain sizes of about 100nm. An extremely sharp superconducting transition with a width of 0.1K and a zero-resistance temperature of 30.3K was obtained, indicating a film of high uniformity and purity in its phase with perfect connection between the MgB2 grains.