Photocatalytic activity of mixed-phase titanium oxide synthesized by sol-gel reaction in supercritical CO2 was investigated with dye degradation of methylene blue. Titanium oxides were synthesized using titanium isopropoxide (TIP), titanium butoxide (TBO), titanium ethylhexoxide (TEO) and titanium diisopropoxide bis (acetylacetonate) (TDB) in supercritical CO2. The photocatalytic activity of titanium oxide synthesized using TEO was higher than that of the commercial titanium oxide P25. The high concentration of TEO precursor in supercritical sol-gel reaction can increase the photocatalytic degradation rate constant of methylene blue in aqueous solutions. Supercritical sol-gel reaction achieved high negative surface charge of the titanium oxide in aqueous solutions, and consequently becomes highly-dispersed. The high dispersion of titanium oxide in aqueous solution results in a large surface area that promotes dye adsorption and photocatalytic degradation.