Abstract: In this study, we employ the Ring Oscillator (RO) based security for the smart grid. Smart grid is the integration of advanced computing and communication technology with the existing power systems. Smart grid can transfer information and data in a bidirectional way. The information exchange provides better management. As the smart grid is much dependent on network it became vulnerable to potential threats. Advanced Metering Infrastructure (AMI) is the part of the smart grid that establishes connections either way. In the present technologies of smart grid, the AMI authentication is done by using the non-volatile memory to store the security keys and using a digital signature or encryption schemes. These methods are costly in terms of area and consume more power. Moreover, memory based authentications are vulnerable to spoofing attacks. In this research, a secure technique independent of non-volatile memory is proposed. This research uses Ring Oscillator Physically Unclonable Functions (ROPUF) on Field Programmable Gate Arrays (FPGA). Instead of storing keys; the circuit derives keys using physical characteristics of FPGA or IC. In this study different levels of security is implemented using ROPUF on FPGA. The keys are modelled from minor feature differences that are occurred at the manufacturing time. So, these keys cannot be modelled, since, irregularities occurred during fabrication process are highly random. In current research, authentication key of programmable sizes is implemented.