We have measured the microwave surface resistance Rs, the magnetic fi eld penetration depth λ(0) and the energy gap Δ(0) of MgB2 thin film s using the dielectric resonator technique. The c_axis textured sample, with superconducting transition temperature of 38K and transition width of 01K, w as prepared on MgO(111) substrates by the chemical vapor deposition method. The Rs value of the MgB2 thin film at 10K was found to be as low as 100 μΩ, which is comparable to that of a high_quality high_temperature thin film o f YBCO. BCS fits of the temperature dependence of Δλ of the MgB2 t hin film gave λ(0)=102nm and Δ(0)=113kTc.