White organic light-emitting device (WOLED) with Sm:Ag black cathode was first reported. The effect of the black cathode on the performances (such as efficiency, luminance, contrast ratio at different angle and EL spectra, etc) of the WOLEDs was discussed. We found that the efficiency of the WOLED with Sm:Ag was comparable to that of the device with conventional Mg:Ag cathode. For example, the efficiency of the device with Sm:Ag cathode is just 15% lower than that of the device with Mg:Ag cathode. However, the contrast ratio (CR) of the device with black cathode is 105:1 under 140 lx ambient lighting at a high brightness of 1000 cd/m(2), which is four times better than that of the device with Mg:Ag cathode. Besides, the CR of the WOLED with Sm:Ag cathode is insensitive to the viewing angle less than 50 degrees .