Abstract: A scan chain designing method includes: obtaining test points according to a gate-level netlist; determining integers M and N, wherein M and N are no greater than an amount X of the test points; selecting M and N test points to be a first and second set test points according to a priority; obtaining a first test coverage and a first test pattern count according to the first set test points and obtaining a second test coverage and a second test pattern count according to the second set test points; obtaining a predicted test coverage curve according to the first and second test coverages; determining an optimum amount O according to the predicted test coverage curve, the first and second test pattern counts, wherein O is no greater than X; and selecting O test points to arrange a scan chain according to the priority and the optimum amount O.