This paper presents an efficient approach based on OBDD for the reliability analysis of a multi-state system subject to imperfect fault-coverage with combinatorial performance requirements. Since there exist dependencies between combinatorial performance requirements, we apply the multi-state dependency operation (MDO) of OBDD to deal with these dependencies in a multi-state system. In addition, this OBDD-based approach is combined with the conditional probability methods to find solutions for the multi-state imperfect coverage models. Using conditional probabilities, we can also apply this method for modular structures. The main advantage of this algorithm is that it will take computational time that is equivalent to the same problem without assuming imperfect coverage (i.e. with perfect coverage). This algorithm is very important for complex systems such as fault-tolerant computer systems, since it can obtain the complete results quickly and accurately even when there exist a number of dependencies such as shared loads (reconfiguration), degradation and common-cause failures.