Photocatalytic degradation of methyl orange in water was examined using Co-doped TiO2 nanoparticles. These photocatalysts were synthesized by a sol–gel method from titanium tetra-isopropoxide with different concentrations of Co(III) dopant and calcination temperature at 600°C. The samples were characterized by XRD, DRS, TEM analysis. The XRD results showed that the anatase-to-rutile phase transformation was greatly inhibited by Co ion doping. The increase of Co doping enhanced “red-shift” in the UV-vis absorption spectra. The TEM images confirmed the dopants suppressed the growth of TiO2 grains. The photocatalytic activity of the nanoparticles under UV light was investigated by measuring the photodegradation of methyl orange solutions. Parameters affecting photocatalytic process such as the dosage of catalyst, dopant and methyl orange concentrations were investigated. The degradation process was optimized using a Co concentration of 1.0 mol%, 30mg Co-doped TiO2 and MO concentration of 10mg/L.