Abstract: Properties of redundant faults in practical circuits are studied in this papers. All （550） redundant faults in ISCAS 85 benchmark combinational circuits are analyzed elaborately. They are divided into five parts. The study shows that only three of them are hardest-to-test faults, and they need much backtrack in algorithm. The rest （547） faults need not any backtrack. Tests can be generated according to intrinsic objective law of circuit’s structure and tests themselves.