Meso-porous Si–N co-doped nano-spherical anatase TiO₂ with high thermal stability and good photo-catalytic activity was successfully synthesized using the hydrothermal method. Mono-dispersed amorphous Si-doped TiO₂ nano-spherical micelles were produced first, through slow hydrolysis of tetrabutyl-orthotitanate (TBOT) and tetraethyl-orthosilicate (TEOS) at room temperature. Then, they were hydrothermally treated in a C₂H₅OH/H₂O suspension using hexa-methylene-tetramine (HMT) as a nitrogen source. The final product was obtained after thermal treatment. The successful incorporation of Si and N into the TiO₂ lattice was confirmed by EDS, FTIR and XPS analyses. The synergetic effect of Si and N plays an important role in improving the photo-catalytic activity. The experimental results and the first-principles density functional calculations showed that the incorporation of N enhances the visible-light-driven photo-catalytic activity, while the incorporation of Si stabilizes the anatase and prevents the oxidation of N at high temperatures.