An organic light emitting display and a method of compensating for a threshold voltage thereof are disclosed. The organic light emitting display includes a display panel including a plurality of pixels, a gate driving circuit generating first and second threshold voltage sensing gate pulses, a data driving circuit which supplies a threshold voltage sensing data voltage to the pixels in response to the first threshold voltage sensing gate pulse and detects a source voltage of a driving thin film transistor (TFT) of each pixel as a sensing voltage in response to the second threshold voltage sensing gate pulse, and a timing controller which modulates input digital video data for the image display based on a change in the sensing voltage and generates digital compensation data.