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Abstract An insulator-to-metal transition (IMT) is an emergent characteristic of quantum materials. When the IMT occurs in materials with interacting electronic and lattice degrees of freedom, it is often difficult to determine if the energy gap in the insulating state is formed by Mott electron–electron correlation or by Peierls charge-density wave (CDW) ordering. To solve this problem, we investigate a representative material, vanadium dioxide (VO2), which exhibits both strong electron–electron interaction and CDW ordering. For this research, VO2 films of different thicknesses on rutile (001) TiO2 substrates have been fabricated. X-ray diffraction (XRD) data show that ultrathin VO2 films with thickness below 7.5 nm undergo the IMT between rutile insulator below T c and rutile metal above T c, while an ultrathin VO2 film with a thickness of 8 nm experiences the structural phase transition from the monoclinic structure below T c to the rutile structure above T c. Infrared and optical measurements on a film of 7.2 nm thickness, below T c, reveal the energy gap of 0.6 eV in the rutile insulator phase and the absence of the 2.5 eV bonding-antibonding CDW structure. Above T c, a Drude feature in the optical conductivity reveals the IMT to a metallic phase. These results suggest that for VO2 films below a critical thickness of about 7.5 nm, the IMT occurs in the rutile structure of VO2 without the Peierls lattice distortion.