The robustness of modern power systems depends on the level of cyber security against cyber-attacks. Since energy transactions could significantly affect the power system operation, these transactions should be evaluated through a secure system to enhance the power system reliability. In this paper, a blockchain-based energy transaction framework is introduced and the level of security for different energy transaction frameworks are evaluated by calculating the overall probability of successful attacks (PSA)to each one. PSA is defined as a successful attack to the system which can lead to the system collapse. The numerical results will demonstrate that the blockchain based energy transaction framework is the most reliable system against cyber-attacks comparing with traditional centralized and modified decentralized energy transaction frameworks.