With consideration of the computation efficiency and unreliable nodes, this paper proposes an efficient network reliability computation method based on isomorphic judgment. For efficiency improvement, this method makes use of CMP (characteristic mergence partition) to identify isomorphic subnet during network decomposition. With CMP, the repeated computations from isomorphic subnet are avoided, and the reliability computation based on OBDD (Ordered Binary Decision Diagram) is enhanced. For unreliable nodes, this method operates the OBDD variables of edges with the edge replacements, which store the unreliable nodes in efficient OBDD structure. Experiment shows this method can accurately calculate network reliability and quickly analyze some networks with many unreliable nodes.