A simple laser ablation technique was used to prepare a stable colloidal TiO2 suspension in pure water. A transparent TiO2 aqueous solution was obtained within a few minutes and its photoactivity for the degradation of methylene blue was measured to be higher than that of commercial TiO2 nanoparticles. SEM analysis revealed that the average size of the nanoparticles increased from 20 to 40 nm as the laser power was raised from 0.5 to 2W. The variation in size, however, had little influence on the resulting photodegradation rate under the given condition. Instead, the photodegradation rate is related to the number of colloidal TiO2 particles in the aqueous solution, which increases proportionally to the ablation time. As the TiO2 particle density increases, however, the photoactivity is measured to be gradually reduced due to the formation of TiO2 aggregates. Thus, the optimum ablation time is 10-30 min under our ablation condition. Our results show that well-dispersed small TiO2 nanoparticles of about a few tens nm can be readily formed by laser ablation within only a few minutes and can be used as highly efficient photocatalysts for photocatalytic remediation of water.