Dispersions of oleic acid coated nano-TiO2 particles/MMA and oleic acid coated nano-TiO2 particles/PMMA were prepared. Their optical absorption was systematically investigated. Results showed that the optical absorption is closely associated with the TiO2 concentration in the dispersion. When the TiO2 concentration (C-0) less than or equal to 0.032 mol%, dispersions of the oleic acid coated TiO2/MMA did not precipitate after several weeks, and the diameters of TiO2 particles were very small (5-10 or 30 nm). For C-0 > 0.032 mol%, the diameters of the particles ranged in two size distributions: 5-10 or 30 nm and 50-80 or 180 nm. Nano-TiO2 particles in dispersions gave rise to ultraviolet (UV) absorption. For C-0 less than or equal to 0.032 mol%, only one UV absorption band was observed, while for C-0 > 0.032 mol%, the absorption band was split into two bands. During polymerization of the dispersion of oleic acid coated nano-TiO2 particles/MMA, the absorption band showed a red shift. These red shifts were due to the oleic acid coating on the TiO2 particles. The origin of the absorption band and the mechanisms of the red shifts of the absorption band are discussed in detail.