The vertically-aligned GaN nanowires (NWs) were grown on Au coated alpha-Al2O3(001) and GaN(002)/alpha-Al2O3(001) substrates by metal organic chemical vapor deposition technique using trimethygallium and tertiarybutylhydrazine (TBHy) as sources. The growth tempearuture of the GaN NWs were reduced to under 700 degrees C due to the low decomposition temperature of TBHy. The vertically-aligned GaN NWs were grown by controlling diameter of Au droplets in the nucleation stage. The GaN NWs grown on alpha-Al2O3 exhibited (101) preferred orientation, while GaN NWs on the GaN(002)/alpha-Al2O3(001) substrate retained [0001] growth direction.