A method of measuring on-resistance in a backside drain wafer includes providing a wafer having a first MOS transistor and a second MOS transistor each having a source and also sharing a drain provided at a backside of the wafer, and then forming a current flow path passing through the first and second MOS transistors, and then measuring a resistance between the sources of the first and second MOS transistors. Accordingly, an on-resistance in a backside drain wafer can be measured without using a chuck.