A TiO₂/CNT nanohybrid photocatalyst is synthesized via sol-gel route, with titanium (IV) isopropoxide and multi-walled carbon nanotubes (MWCNTs) as the starting materials. The microstructures and phase constitution of the nanohybrid TiO₂/CNT (0.005wt%) samples after calcination at 450°C, 550°C and 650°C in air are compared with those of pure TiO₂ using field-emission scanning electron microscopy and X-ray diffraction, respectively. In addition, the photocatalytic activity of the nanohybrid is compared with that of pure TiO₂ with regard to the degradation of methyl orange under visible light irradiation. The TiO₂/CNT composite exhibits a fast grain growth and phase transformation during calcination. The nanocomposite shows enhanced photocatalytic activity under visible light irradiation in comparison to pure TiO₂ owing to not only better adsorption capability of CNT but also effective electron transfer between TiO₂ and CNTs. However, the high calcination temperature of 650°C, regardless of addition of CNT, causes a decrease in photocatalytic activity because of grain growth and phase transformation to rutile. These results such as fast phase transformation to rutile and effective electron transfer are related to carbon doping into TiO₂.