Graphene oxide(GO) was prepared by modified Hummers method utilizing natural graphite powders. The obtained GO was dispersed in water and broken into small pieces with different sizes by ultrasonication. Then, the GO dispersion was separated by centrifugation. At last, more uniform graphene oxide nanosheets were obtained. Through statistical analysis of the size of GO, based on vast of field emission scanning electron microscope (FE-SEM) images, it is can be found that the size of GO could be effectively controlled by adjusting ultrasonication power, ultrasonication time, centrifugation speed and centrifugation time. With the optimized parameters of ultrasonication-centrifugation (UC) process, GO nanosheets with an average size of about 100 nm were synthesized, and the content of GO nanosheets smaller than 100 nm was more than 60%.