We combined two different approaches for improving activity of TiO2 photocatalyst; band gap narrowing by hetero-valent dopant and improving electron-hole separation rate by band offset at the anatase-rutile boundary. Polymorphic TiO2 particles are usually synthesized by heating, and particles inevitably grow. Hetero-valent elements, such as W, doping reduces the band gap of TiO2, and improving photon absorption efficiency. However, it also make space charge layer near the surface thinner which can reduce electron-hole separation rate. Therefore, it is difficult task to get synergetic effects of band gap narrower and high carrier separation rate of polymorphic particles. We achieved this by co-doping of tungsten, a band gap narrower, and tin, a promoter of anatase-to-rutile transformation (ART). By W-Sn co-doping, we synthesized ~ 10 nm sized nanoparticles of anatase-rutile mixed phase. The photo-catalytic activity was compared with solely doped TiO2 and commercially available P25. We also reduced temperature by 50 K, which can save large amount of energy.