Light-emitting diodes based on heterojunction with structure ITO/PDDOPV/PPQ/Al have been successfully fabricated, in which PDDOPV {poly (2,5-bis (dodecyloxy)-phenylenevinylene]} is a hole transporting polymer while PPQ } is a hole transporting polymer while PPQ [poly (phenyl quinoxaline)] is an electron transporting polymer. Light emission was observed under both positive and negative bias voltage. Light emission under positive bias voltage mainly is originated from PDDOPV, but light emission under negative bias voltage includes blue light emission which is from PPQ and yellow emission which is from PDDOPV. Intensity ratio of the blue emission to yellow emission increases with increasing negative bias, when the reverse applied voltage is 22 V, 24 V, 26 V, 28 V, respectively, the ratio of PPQ peak value to PDDOPV peak value, IPPQ/IPDDOPV are 1.092, 1.329, 1.605, 2.046 respectively. In other words, the emitting color is voltage-dependent color tunable, which is a promising property to realize colorful display. The light emitting mechanism under negative bias voltage is analyzed.