Anatase TiO2 nanocrystal colloids with high dispersion and photocatalytic activity were rapidly synthesized from peroxo-titanium-acid precursor by microwave-assisted hydrothermal method within 30 min at low temperature (120-180 A degrees C). The transmission electron microscopy results indicate that the as-prepared TiO2 have a narrow particle size distribution (25-29 nm) and high dispersion. The crystal structure of all these products are pure anatase phase (XRD, Raman), and they show good crystallinity and large surface area (N-2 adsorption-desorption measurements BET). The results of the UV-Visible absorbance and Fourier transform infrared spectra indicate that the surface peroxo group Ti(O-2) still remains in TiO2 nanoparticles prepared by microwave-assisted hydrothermal method at 120 A degrees C, and this surface peroxo group can be decomposed effectively by drying at 140 A degrees C. The photocatalytic activity of the as-prepared TiO2 were evaluated by the degradation of reactive brilliant red X-3B, it is found that the as-prepared TiO2 exhibited good photocatalytic performance. Moreover, the existence of surface peroxo group greatly suppressed the photocatalytic activity of the TiO2 nanoparticles.