Abstract: A reliability analysis model was developed for the series system with spare parts to solve the reliability index of complex mechanical systems. The system can be transformed into an equivalent one constructed with cold standby systems in series. When the life span and repair time of the parts obey exponential distributions, the system can be described by the Markov process. In the case of correlative life and irrelative repair time distributions and the failure parts can be repaired as good as new, the system model was built. The formulae of the system availability, reliability and mean time to first failure were derived by solving the state transition probability matrix. The results showed the model is suitable for unrepairable systems too.