In this paper, we present a heuristic method to reduce fault candidates for an efficient fault diagnosis. This paper uses a matching algorithm for the exact fault diagnosis. But the time consumption of a fault diagnosis using the matching algorithm is huge. So, we present a new method to reduce the fault diagnosis time. The method to reduce the time consumption is separated into two different phases which are a pattern comparison and a back-tracing comparison in failing pattern. The proposed method reduces fault candidates by comparing failing patterns with good patterns during critical path tracing process and comparing back-tracing from non-erroneous POs with backtracing erroneous POs. The proposed method increases the simulation speed than the conventional algorithms. And this method is also applicable to any other fault diagnosis algorithms. Experimental results on ISCAS'85 and ISCAS'89 benchmark circuits show that fault candidate lists are reduced than those of previous diagnosis methods.