An organic light emitting display which includes a display panel having a pixel cell formed in a region defined by gate lines and data lines perpendicularly crossing each other, a power supply which supplies current to the display panel, a scan driver which supplies a scan signal to a gate line, a data driver which supplies a data voltage to a data line, a timing controller which supplies a control signal to the scan driver and the data driver and an converted pixel data signal to the data driver, a gradation converter which converts a gradation of a pixel data signal inputted and supplies the converted pixel data signal to the timing controller, and a scale parameter generator which generates a scale parameter through the converted pixel data signal and supplies the scale parameter to the gradation converter, when the next pixel data signal is inputted to the gradation converter.