An organic light emitting display device includes: pixels including driving transistors positioned in regions divided by scan lines and data lines; a data accumulating unit arranged to accumulate first data; a first storage unit storing current and voltage change information corresponding to a degradation of an organic light emitting diode (OLED); a second storage unit storing a compensation value corresponding at least partially to channel length modulation of the driving transistors; and a timing controller programmed to carry out an altering of first data corresponding to an ith pixel so as to generate second data to be supplied to the ith pixel, the altering carried out according to: accumulation stress information for the ith pixel, the accumulation stress information corresponding to the accumulated first data and being stored in the data accumulating unit, the current and voltage change information, and a compensation value corresponding to the ith pixel.