The phase transition process from monoclinic VO 2 (B) to monoclinic rutile-type VO 2 (M) nanorods under hydrothermal conditions was explored. VO 2 (M) nanorods were formed by nucleating nanobumps on the surface of VO 2 (B) nanorods and growing them until the VO 2 (B) transformed entirely to VO 2 (M). The metal–semiconductor transition temperature of VO 2 (M) nanobumps exhibited an extremely strong size effect. The critical size for the nano-size effect was determined to be <13.0 nm; below this size, phase transition temperatures were significantly reduced from 68.0 °C for bulk VO 2 (M) to 5.0—17.0 °C and from −14.5 to −11.0 °C for heating and cooling semicycles, respectively. These results are attractive because they indicate a new strategy to regulate the phase transition temperatures purely through the particle size.