A simple, cost-effective approach is presented for producing exfoliated films of pure graphene or polymer-graphene composite with high yield, high conductivity, and processability. The approach combines supercritical CO2 with ultrasonics. Characterization by Raman spectroscopy combined with atom force field microscopy demonstrates that the graphene sheets were obtained with 24% as monolayers, 44% as bilayers, and 26% as trilayers. The layer number and lateral size of graphene sheets can be controlled by adjusting the process parameters. The yield of graphene sheets with a lateral size of about 0.5-5.0 mu m is about 16.7 wt % under optimum conditions, which can be easily raised to 40-50 wt % by repeated exfoliation of the sediment that remained in the reactor. The resultant pure graphene film made by filtration has a high electrical conductivity of 2.8 X 10(7) S/m. The electrical conductivity of the film of polyvinyl alcohol graphene composite is 300 S/m.