We demonstrated that manganese (Mn)-doped GaN nanowires (NWs) exhibit p-type characteristics using current-voltage (I-V) characteristics in both heterojunction p-n structures (GaN:Mn NWs/n-Si substrate) and p-p structures (GaN:Mn NWs/p-Si). The heterojunction p-n diodes were formed by the coupling of the Mn-doped GaN NWs with an n-Si substrate by means of an alternating current (AC) dielectrophoresis-assisted assembly deposition technique. The GaN:Mn NWs/n-Si diode showed a clear current-rectifying behavior with a forward voltage drop of 2.4 V to 2.8 V, an ideality factor of 30 to 37, and a parasitic resistance in the range of 93 kX to 130 kX. On the other hand, we observed that other heterojunction structures (GaN:Mn NWs/p-Si) showed no rectifying behaviors as seen in p-p junction structures.