TiO2 nanoparticles capped with a layer surfactant were prepared by colloid-chemical method, and their structural phase transformation and optical absorption properties were investigated. XRD and TEM results showed that the surfactant capping effectively inhibits the grain growth during heat-treatment, and results in a different crystallization behavior in which titania nanoparticles with amorphous structure can simultaneously transform into anatase and rutile when heat-treated below 400℃. It was found from optical absorption measurement that the capped TiO2 colloidal particles in toluene sol have a large red shift of absorption band edge in contrast with that without capping, which is mainly attributed to the effects of the interface dipole and dielectric confinement. In the investigation of the relation between the absorption coefficient and photon energy, it was found that there exist two linear correspondences of (αhν)1/2 vs hν(indirect transition) and (αhν)2 vs hν (direct transition) for the titania films and colloidal-particle organic sol. The respective band gap values were obtained by extrapolation of the above linear relation.