In an organic light emitting display and its driving method, a pixel portion includes a plurality of pixels which express images corresponding to a scan signal, an emission control signal, and a data signal. A scan driver transfers the scan signal and the emission control signal to the pixel portion. A data driver generates and transfers a plurality of data signals to the pixel portion using video data. A frame memory stores and transfers the video data in frame periods to the data driver. A luminance controller determines a pulse of the emission control signal using frame data, which is a sum of video data stored in the frame memory. A power supply unit supplies voltages of first and second power sources to the pixel portion. The luminance controller determines the number and widths of pulses in the emission control signal corresponding to a sum of the video data.