Large scale production of graphene nanosheets was achieved by arc evaporation of a graphite rod in air. The graphene nanosheets are approximately 100-200 nm wide and the number of layers is mainly in the range of 2-10. Several tens of grams of product were obtained per day. The yield of graphene nanosheets was found to be dependent on the pressure of the air, i.e. high pressure facilitates the formation of graphene nanosheets, but low pressure favors the growth of other carbon nanostructures including carbon nanohorns and nanospheres. Based on this result, a pressure-induced mechanism of formation of graphene nanosheets is proposed. The impurities in the products could be eliminated by oxidation in air.