The present disclosure relates to a method for detecting a defect of a sensing transistor disposed in each sub pixel of an organic light emitting display device and extracting a coordinate of a sub pixel in which a defective sensing transistor is disposed. A defect detecting data voltage is applied to a data line after initializing a reference voltage line and a voltage of the reference voltage line is sensed during an interval in which the scan transistor is turned on and the sensing transistor is turned off to determine whether the sensing transistor is defective.