We report the Raman results obtained from single GaN, GaN/AlN core-shell, and GaN/AlN branched nanowires (NWs). Polarized Raman spectra from a single GaN NW showed strong anisotropic behavior, in agreement with the Raman polarization selection rules for a wurtzite crystal, indicating the high crystalline quality of the NW. A Raman spectrum from a single GaN NW revealed several optical phonons, including A(1)(TO), A(1)(LO), and E-2(H) phonons at 530.5, 724.2, and 567.0 cm(-1), respectively. Fabricating an AlN shell layer on the side wall of the GaN core NW shifted the GaN A(1)(LO) phonon energy upward by 5.7 cm(-1), indicating that compressive strain occurred in the GaN core. The formation of AlN nanorod branches on the GaN/AlN core-shell surface shifted the A(1)(LO) phonon energy downward toward the value of the GaN NW, indicating a relaxation of the compressive strain in the GaN core. In particular, a broad phonon response was observed at 691.8 cm(-1) on the low-energy shoulder of the GaN A(1)(LO) phonon peak. A careful analysis of this mode identified that the 691.8 cm(-1) mode corresponded to a defect-related phonon, not to a surface optical phonon.