Thin films of amorphous indium gallium zinc oxide (a-IGZO) were fabricated by DC magnetron sputtering. The influence of sputtering pressure on the microstructures and the electronic properties were investigated in details. AFM characterization on surface morphology demonstrated that the surface roughness increases with the sputtering pressure. The oxygen vacancies of the a-IGZO films changes considerably and were reduced significantly with increasing sputtering pressure, as disclosed by X-ray photoelectron spectroscopy. Both the increased surface roughness and reduced oxygen vacancy are detrimental to the performance of a-IGZO TFTs. From this point of view, the sputtering should be done at a proper pressure of 0.06 Pa in order to ensure the enhanced performance. Specifically, the electron saturation mobility (μsat) and the threshold voltage (VTH) of the a-IGZO TFTs are 3.32 cm2/(V?s) and 24.6V at such a sputtering condition.