Two key indices in system reliability evaluation are the probability and frequency of failure. Other measures such as, the mean cycle time and the mean down time can be easily derived from these quantities. This paper deals with the reliability evaluation of a complex maintainable system using a cut set approach. The available literature on this subject generally gives the probabilities of failure. Buzacott⁽³⁾ has given a method to determine the frequency of failure but his method has the drawback that explicit formulae for system availability must be derived and numerical values are then obtained by further manipulation. This approach is, therefore, not suitable for computer application. The contribution of this paper is in developing a new formula for the failure frequency using cut set approach, wherein the numerical values can be directly obtained. This method therefore overcomes the drawback of Buzacott's method and is suitable for computer application. An upper bound approximation for frequency is also given and the method is illustrated by an example.