Smart grid is the next generation intelligent power grid that combines the existing electric power infrastructure and information infrastructure. It can optimize the energy efficiency in both directions, suppliers and power consumers to exchange information in real time. In smart grid environments, with existing network security threats due to the smart grid characteristics, there are additional security threats. In this paper, we propose a security mechanism that provides mutual authentication and key agreement between a remote user and the device. The proposed mechanism has some advantages that provides secure mutual authentication and key agreement and secure against a replay attack and impersonation attacks.