We employed ab initio calculations to study the effect of various defects on the structural, electronic and optical properties of anatase TiO 2. Single Cr doping at Ti sites introduced isolated Cr 3d states in the band gap of TiO 2. Charge compensated Cr doped model was introduced by simultaneously doping two Cr atoms at Ti sites along with one oxygen vacancy ( Ti 14 O 31 Cr 2). It has been found that Ti 14 O 31 Cr 2 can narrow the band gap significantly and passivate gap states. Removal of the isolated Cr 3d states from the band gap and improved visible light absorption would effectively enhance the photocatalytic activity of Cr doped TiO 2. Our calculations provide reasonable explanation for the previously reported experimental findings.