The current crowding effect induced by oxygen adsorption in InGaZnO thin film transistors was investigated by capacitance-voltage measurements. During a drain-bias stress, the oxygen was adsorbed on the back channel near the drain region of InGaZnO film and formed a depletion layer, which lead to an increase in the parasitic resistance near the drain region. Moreover, the adsorbed oxygen caused the capacitance value to increase in the gate-to-source capacitance and the subthreshold capacitance to exhibit a stretch-out phenomenon in the gate-to-drain capacitance. Therefore, using the capacitance-voltage measurements supports the oxygen adsorption mechanism which induces drain current degradation after drain-bias stress. Crown Copyright (c) 2012 Published by Elsevier Ltd. All rights reserved.