With a high surface-to-volume ratio, a Ga-polar GaN nanorod array was designed and obtained as a precursor for growing ferromagnetically enhanced GaN:Mn film by MOCVD. HRXRD and Raman scattering results might imply a correlation between the ferromagnetism in GaN:Mn and built-in defects in the intrinsic GaN lattice, which were produced when Mn doping was carried out.