The surface properties of graphene have a significant impact on its electrical characteristics. In this paper, an approach to change electrical characteristics of graphene by various gases in atmosphere is reported. It is found that the P type impurities on graphene are reduced and transconductances of graphene transistors are enhanced when nitrogen gas, argon gas, oxygen gas or compressed air is blown on the surface of graphene. These phenomena are attributed to the decrease of the surface impurities and carrier scattering rates on the graphene. These results are useful in graphene sensors, devices, and circuits.