The phase transition process from monoclinic VO2 (B) to monoclinic rutile-type VO2 (M) nanorods under hydrothermal conditions was explored. VO2 (M) nanorods were formed by nucleating nanobumps on the surface of VO2 (B) nanorods and growing them until the VO2 (B) transformed entirely to VO2 (M). The metal-semiconductor transition temperature of VO2 (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 degrees C for bulk VO2 (M) to 5.0-17.0 degrees C and from -14.5 to -11.0 degrees 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. (C) 2010 Elsevier B.V. All rights reserved.