Abstract: Current critical path tracing algorithm will lead approximation of results in fault diagnosis, since self-masking and multiple path sensitization in determining criticality of fan-out stem are not considered. The present paper studies the effects of self-masking and multiple path sensitization on determining criticality of fan-out stem, and the accuracy of the algorithm is improved. Based on an 18-valued algebra in symbolic simulation, stuck-at faults and transition faults can be detected simultaneously, and the efficiency of the algorithm is improved. Critical path tracing algorithm, considering self-masking and multiple path sensitization in determining the criticality of fan-out stem, is implemented with the C++ language. The improved algorithm is applied to ISCAS'85 standard circuits for fault diagnosis, and experimental results prove the accuracy and efficiency of the algorithm.