A copper(Cu) thick film conductor containing glass and metal oxide for aluminum niride(AlN) substrate was developed. The conductor showed adhesion strength and reliability which were almost comparable to those of Ag-Pd conductors and also had good solder wettability and erosion properties. The Cu conductors must be fired in a nitrogen atmosphere containing oxygen gas. When they were fired under a low oxygen concentration, the gasses thermally decomposed and their properties changed which meant that the molten gasses could not flow smoothly to the AlN surface, so adhesion strength decreased. On the other hand, under high oxygen concentration, the adhesion strength increased because the thermal decomposition and property changes were suppressed. However, poorer solder wettability was brought about because copper was oxidized. Metal oxide added to the conductor could improve the wettability without decreasing the adhesion strength, even if it was fired at the higher oxygen concentration. Suitable metal oxides were CdO, Co3O5 and Fe2O3.