The effects of annealing temperature, annealing time and film thickness on the properties of MgB2 thin films prepared by ex-situ annealing of Mg/B multilayer have been studied. The films with the thickness of 250 nm annealed at temperatures as low as 400 ℃ exhibited a superconducting transition. The optimal annealing condition of 250 nm thick MgB2 films regarding to the transition temperature, Ｔc, was 750 ℃ for 20—30 min. Ｔc became lower as the film thickness decreased and the Ｔc of thinner MgB2 films was influenced more greatly by the annealing temperature. With reducing the thickness of each single layer, we were able to fabricate the 20 nm thick MgB2 superconducting film with a Tonsetc of 31K.