The preparation of graphene oxide (GO) sheets with specified size was developed by simply controlling the time of ultrasonication to the large-size GO (LGO) sheets. The LGO sheets were synthesized by choosing large parent graphite, mild oxidation condition and a two-step centrifugation. The different-sized GO samples prepared under different ultrasonication times, are characterized by Scanning electron microscopy, X-ray photoelectron spectroscopy, Ultraviolet-visible spectroscopy, and X-ray diffraction. It is found that the size of the GO sheets, which has a Gaussian distribution, decreases from 231 to 17 mu m(2) as the ultrasonication time increases. Moreover, the ultrasonication not only can exfoliate and break GO sheets, but also increase the oxidation degree of GO sheets, especially when the GO sheets have a weak oxidation degree. It is reasonable to believe that the size of GO sheets is closely correlated to the C-O content, which enables the size of GO sheets to be controlled. Our work demonstrates that ultrasonication is an important method to control the size and the oxidation degree of GO sheets, to a certain extent.