Abstract: Five kinds of single layer OLED light emitting devices with different thicknesses of Alq₃ light emitting layer were fabricated by vacuum evaporation method. The effect of different thicknesses of the Alq₃ light emitting layer on the luminous performance and paramenters of single-layer OLED light emitting devices were studied. It is found that in single layer OLED devices, with the increase of the thickness of the light emitting layer, the ingression of luminescence region causes the excition to move further away from the poles, which weakens the quenching effect and improves the current efficiency. However, the thickness of the light emitting layer Alq₃ is not the thicker the better. With the increase of the thickness, the driving voltage is also increasing and the performance is weakening. When the current density is less than 200 mA/cm², the device with 250 nm thick Alq₃ light emitting layer has the highest brightness and the best performance.