We report the size effects on the metal-insulator phase transition of vanadium dioxide (VO2) nanowires prepared by chemical vapor deposition. The phase transition temperature can be tuned from 67 °C in the bulk VO2 to as low as 29°C by reducing the diameter of VO2 nanowires to nanoscale. Temperature-dependent Raman spectra display a clear dynamic picture on the metal-insulator phase transition process of the VO2 nanowires. Whilst, Raman study shows no remarkable strain effect on the phase transition behaviors of our samples. The increasing surface defect density with reducing nanowire size facilitates the decreasing phase transition temperature. In addition, the polarized-photocurrent effect was observed, resulting from the anisotropy of the photoresponse and also caused by the reduced dimensionality.Our results indicate that size of VO2 nanostructures can dominate their thermoelectric and photoelectrical properties.