Abstract: Taking a manufacturing unit having three machines for simultaneous and multitask machining jobs as example, extended stochastic Petri nets (ESPN) is introduced to develop a multitask reliability model adaptable to such manufacturing systems that process time are arbitrary distributions. In addition, the model rises above the difficulty that the time-dependent dynamic behavior in manufacturing process cannot be involved by way of conventional reliability modeling. For the reliability model of complex manufacturing systems, the concept of moment generating function is introduced in the behavior expression so as to obtain the multitask reliability of the whole system by computing the transfer functions of the model without generating a reachability graph. In this way the performance in service of the whole manufacturing system for multitask machining jobs can be perceived clearer.