The electronic structure across the metal-insulator (MI) transition of electron-doped V 1−x W x O 2 epitaxial films (x = 0−0.06) grown on α-Al 2 O 3 substrates was studied by means of thermopower (S) measurements. Significant increase of |S| values accompanied by MI transition was observed, and the transition temperatures of S (T S ) decreased with x in a good linear relation with MI transition temperatures. |S| values of V 1−x W x O 2 films at T > T S were constant at low values of 23 μV K −1 independently of x, which reflects a metallic electronic structure, whereas those at T < T S almost linearly decreased with logarithmic W concentrations. Vanadium dioxide (VO 2 ) has attracted considerable attention due to its ability to reversibly transform from a lowtemperature insulator into a high-temperature metal at ß340 K [1]. The metal-insulator (MI) transition is accompanied by a structural change from monoclinic to tetragonal-type rutile structure. The transformation of crystal structure originates from dimerization of vanadium ions with accompanying the position shifting from linear chains along c axis of rutile phase to zigzag type, resulting in a monoclinic structure. The structural change causes reconstruction of electronic structures to open up a charge gap of ß0.6 eV that abruptly changes both the electrical resistivity and infrared transmission [2]. These features of the MI transition for VO 2 appear promising for potential applications to electrical and optical switching devices, operating at room temperature (RT). Recently, reversible alternation of electronic properties from insulator to metal state was demonstrated by both electrostatic charge doping [3] and hydrogenation [4], which enables on-demand-tunable devices using the MI transition of VO 2 .However, the driving mechanism of the MI transition in VO 2 is still not fully understood, i.e., it has been debated that the MI transition should be regarded as either a structurally driven Peierls transition with electron-phonon interaction or a Mott transition with strong electron-electron correlation [5]. Thus, intensive efforts have been devoted to experimentally observe electronic structure change of VO 2 across the MI transition mainly by spectroscopic techniques, such as x-ray photoemission spectroscopy (PES) [6] and angle resolved PES [7] for valence band structure observation, as well as x-ray absorption spectroscopy [8] for the conduction band structure, but the mechanism of the MI transition is still unclear. Further investigation on the electronic-structure evolution by another experimental means is inevitable for the elucidation of MI transition, which should give crucial information for fundamental physics as well as for practical device application of VO 2 .Here we focused on thermopower (S) as a physical property to investigate the electronic structure across the MI transition, because S values should be sensitive to significant changes in * Corresponding author: hiromichi.ohta@es.hokudai.ac.jp the electronic structure of VO 2 at T MI . In g...