An OLED including an anode layer, a first mixing layer over the anode layer, a mixing layer on the first mixing layer, a second mixing layer on the mixing layer, and a cathode layer over the second mixing layer is provided. The mixing layer is a mixture of an organic light emitting material, a hole and an electron transport material, and a volume ratio of the hole transport material to the electron transport material thereof is X %. The first and second mixing layers are both mixtures of the hole and electron transport materials. A volume ratio of the hole transport material to electron transport material in the first mixing layer decreases gradually from 99% to X % starting from the surface adhered to the anode layer, and that of the second mixing layer increases gradually from X % to 99% starting from the surface adhered to the mixing layer.