A series of experiments on the corona current pulse appearing on insulators under various contamination conditions with AC voltage of different amplitude are described. The corona current pulses were found to follow the Gaussian distribution and can be used to distinguish the faulty insulators from normal insulator strings in transmission line. The probability formulas about the corona current pulses have been studied in detail. They can be used as a kind of criterions to detect the faulty insulators. With several established fuzzy membership functions of humidity and contamination through data from experiments, it is proved that the insulation state could be estimated from the transmission line insulator strings by the fuzzy logical reasoning. This method has been verified in the laboratory by simulating work conditions of insulator strings on an 110kV transmission line and has the advantage of safety and convenience.