A new catalyst seeding method is presented, in which aerosolized catalyst nanoparticles are continuously self-assembled onto amine-terminated silicon substrates in gas phase to realize controllable synthesis of vertically aligned Mg-doped GaN nanorod arrays on n-type Si (111) substrates. The diameter, areal density, and length of GaN nanorods can be controlled by adjusting the size of Au nanoparticles, flowing time of Au nanoparticles, and growth time, respectively. Based on the synthesis of p-type GaN nanorods on n-type Si substrates, p-GaN nanorod/n-Si heterojunction diodes are fabricated, which exhibit well-defined rectifying behavior with a low turn-on voltage of similar to 1.0 V and a low leakage current even at a reverse bias up to 10 V. The controllable growth of GaN nanorod arrays and the realization of p-type GaN nanorod/n-type Si heterojunction diodes open up opportunities for low-cost and high-performance optoelectronic devices based on these nanostructured arrays.