Electroluminescent characteristics of n-ZnO/p-GaN hetero junctions under forward and reverse biases are studied.Emissions at 389nm and 570nm are observed under forward bias. An unusual emission at 390nm appears under reverse bias, and is attributed to the recombination in the p-GaN side of the hetero junction. The yellow emission peaked at 570nm is suppressed under reverse bias. The light intensity exponentially depends on the reverse current. The emission under reverse bias is correlated to tunnelling carrier transport in the heterostructure.Our results also support that the well-known yellow band of GaN comes from the transitions between some near-conduction-band-edge states and deep localized acceptor states.