Reliability analysis of a logic network is considered in this paper. A logic network has some special properties that are not very common in reliability engineering. Firstly, the structure function of a logic network depends not only on the work of its components (logic gates), but also on the values of the input signals. Secondly, a logic network is a typical type of noncoherent system. These two characteristics imply that its reliability analysis is more complicated than other systems. In this paper, we study the structure function of a logic network and propose a definition of the Structural Importance Measure (SIM) for this type of systems. The definition of the SIM is based on Logical Differential Calculus.