Light-emitting diodes based on ROPPV-12/Alq3 heterojunction were fabricated, in which ROPPV-12 [poly(2, 5-Didodecyloxy-1, 4-Phenylenevinylene)] was a hole type semiconducting polymer, and 8-hydroxyquinoline aluminum (Alq3) was an electron type semiconducting low mass organic material. The emission spectrum of the device based on heterojunction ROPPV-12 (70 nm)/Alq3 (20 nm) is similar to that of POPPV-12′s single layer device. The emission spectrum of the device based on heterojunction ROPPV-12 (70 nm)/Alq3 (32 nm) is composed of two parts, one is originated from ROPPV-12, the other is from Alq3. The emission from ROPPV-12 is the main emission at low drive voltage. While with the increase of drive voltage, the emission from Alq3 becomes the main emission gradually. In other words, the electroluminescence of the later device is color voltage-dependent. The brightness, current and luminescent efficiency of the former device is much better than the later device. The emission mechanism is presented.