This paper presents an efficient method to evaluate performance of tree-structured assembly/disassembly (AD) systems with finite buffer capacity. The times to failure, the times to repair, and the processing times are assumed to be exponentially distributed. The method is based on an extension of a decomposition method originally applied to performance evaluation of transfer lines. The method decomposes aK-machine tree-structured AD system to a system ofK- 1 two-machine lines and then finds the processing rates, failure rates and repair rates of the decomposed system that make performance of the two systems close to each other. To determine those rates of the decomposed system, we derive 6(K- 1) equations based on interruption of flow, resumption of flow, and flow rate-idle time relations, and suggest two methods, which use the equations in different ways. Results from computational experiments show that the suggested decomposition methods give good estimates in a short time.