An organic light emitting display device including: a substrate having a pixel region and a non-pixel region; a first buffer layer and a second buffer layer formed over the substrate; a thin film transistor formed over the second buffer layer; an organic light emitting device formed in the pixel region and electrically connected with the thin film transistor; and a photo diode formed in the non-pixel region, for receiving external light of red wavelength at a certain absorption rate, and for controlling brightness of the organic light emitting device. Here, the first buffer layer can be formed to be from 2900 Å to 3100 Å in thickness, the second buffer layer can be formed to be from 200 Å to 400 Å in thickness, and the photo diode can include: an N-type doping region, a channel region being from 3 to 10 μm in width, and a P-type doping region.