A method, a test line and a system for detecting defects on a semiconductor wafer are presented. The method includes measuring a current-voltage (IV) curve of a plurality of metal oxide semiconductor (MOS) transistors which are connected in series in a test key; comparing the measured IV curve with a reference curve to obtain a first drain current drop in a linear region and a second drain current drop in a saturation region; and determining whether at least one of the MOS transistor among the MOS transistors of the test key is defected according to at least one of the first drain current drop and the second drain current drop.