Nanocrystalline Gd2O3 doped CeO2 (abbreviated as GDC) ion conductor electrolyte thin films synthesized by reactive magnetron sputtering on (0001) sapphire substrates have been characterized by X-ray diffraction (XRD)，atomic force microscopy (AFM) and AC impedance analysis. The results show that，the f.c.c structured GDC films have strong (111) textures at all the substrate temperatures of our investigation while the surface morphology varied with the temperature. Small, round growth islands transform to large prismatic islands at temperatures from room temperature to 300℃, and the reverse process occurr at temperatures from 400℃ to 700℃. This morphology change characterizing different nucleation mechanisms at the beginning of film growth is probably due to transformation of the surface structure of the (0001) sapphire at different temperatures. The AC impedance complex plane plot of the GDC film is mainly determined by grain boundary resistances. The conductivity activation energy (1.2—1.5eV) calculated by the Arrhenius plot is close to the reported value for the grain boundary conductivity and decreases with rising substrate temperature (Ea300Ea400Ea600). Difference in activation energy and grain size for GDC films causes unequal increasing rates of electrical conductivity at higher temperatures.