Anatase type TiO2 has been previously largely reported as a candidate negative electrode material for lithium-ion batteries. We report here for the first time the complete in situ Raman study of lithium insertion and de-insertion into three variously nano-sized TiO2 anatase powders (Prolabo, ca. 80nm, AK1, ca. 15nm and MTi5 ca. 8nm), of which AK1 and MTi5 show superior capacity and cyclability. From these measurements realized in a galvanostatic mode between 3 and 1V versus Li/Li+, the phase transition from a tetragonal to an orthorhombic structure was clearly observed to take place at different quantities of x in Li x TiO2. These results confirm the extension of the solid solution domain as particle size is reduced. For the smaller TiO2 nano-sized materials (AK1 and MTi5), a more pronounced decrease in band intensity when x >0.3 for Li x TiO2, was observed and may be related to the decrease in the optical skin depth linked to the conductivity increase as lithiation proceeds.