Light-emitting diodes based on heterojunction with structure ITO/PDDOPV/PPQ/Al have been successfully fabricated,in which PDDOPV is a p-type polymer while PPQ is an n-type polymer.Light emission was observed under both positive and negative bias voltage.Light emissionm 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.The ratio of intensity of blue emission to that of yellow emission increases with the growth of negative bias voltage.In another word,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.