A device for sensing a threshold voltage of a driving TFT in an organic light emitting display includes a data drive circuit and a timing controller. The data drive circuit applies a data voltage to a gate node of the driving TFT during a first programming period, determines a source node voltage of the driving TFT as a first sensing voltage during a first sensing period in which a gate-source voltage is constant and higher than the threshold voltage, applies another data voltage to the gate node during a second programming period, and determines the source node voltage as a second sensing voltage during a second sensing period in which the gate-source voltage is constant and higher than the threshold voltage. The timing controller calculates a ratio between the first and second sensing voltages, and obtains a change in the threshold voltage using a change in the ratio.