The band structure and dielectric properties of the pure ZnO and the Al-doped ZnO were studied by using a first-principle ultrasoft pseudopotential approach of the plane wave based on the density functional theory. The pure ZnO and the Al-doped ZnO powders were prepared via the solid state reaction at 600°C with holding time of 1.5 h. The prepared powders were characterized by X-ray diffraction （XRD） and X-ray photoelectron spectroscopy （XPS）. The dielectric parameters were determined by the vector network analyzer in the frequency range of 8.2—12.4 GHz. Results show that the volume of super-cell has no obvious change and the Fermi energy level is introduced into conduction band through introducing Al ions. XRD patterns indicate that all the samples have pure wurtzite structure of ZnO. It is found that Al ions form the substitutional impurity in ZnO crystal according to the result of XPS. The experimental results show that both the real part ε′ and imaginary part ε″ of permittivity of the samples are increased by Al doping, in agreement with the result of calculation.