This paper suggest failure analysis of mechanical systems using function cum structure approach. Operational parameters of system function are identified and their relationships are developed. The predicates, which comprise the components of the system and their properties as their attributes, i.e., type of connectivity (fixed or sliding contact) and functions (transmit torque, and transmit force etc.) represent the facts. The facts are modelled in terms of functional-cum-structure graph. By analysing interaction of a failure function i.e., function of a failed component in the graph with other functions, the failure cause-function is identified. The problem can either be solved by removing the cause-functions or by modifying the failure function. A critical function is also identified from function set on the basis of importance of a function. Appropriate parameters for in-situ design provision for condition monitoring of the system are also identified. The steps of the methodology are included and are illustrated by means of an example.