Performance evaluation techniques of manufacturing systems are powerful decision support tools especially at system design stage. In this paper, an approximate technique based on decomposition method is proposed for performance evaluation of serial homogeneous transfer lines with unreliable machines and finite transfer-delay buffers. The characteristic of transfer-delay buffers means that it takes some transit time to transfer the parts through buffers, which increases the complexity of transfer lines. In order to address the problem, we firstly transform the initial transfer line into an equivalent transfer line, a serial non-homogeneous transfer line with unreliable machines and finite buffers. Then another transformation which replaces the equivalent line by an approximate homogeneous line is introduced. Finally, decomposition method is used to deal with the approximate model. The accuracy and efficiency of the method are validated by simulation results.