Polymer-typed viologen has been blended into the luminescent polymer, poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV), in fabricating the polymer light-emitting diode (LED) in order to overcome the photo-oxidation problem of the LED. The addition of viologen improved the device performance: the stability of the luminescent polymers can be enhanced; the rum-on voltage of the LED with viologen is decreased and the external quantum efficiency of its electroluminescence is increased compared with the LED without viologen. The preliminary result of the a.c. impedance measurements indicate that the addition of viologen can improve the stability of the LED against the UV radiation.