ZnO quantum dots dispersed in a silica matrix were synthesized from a TEOS:Zn(NO3)(2) solution by a one-step aerosol gel method. It was demonstrated that the molar concentration ratio of Zn to Si (Zn/Si) in the aqueous solution was an efficient parameter with which to control the size, the degree of agglomeration, and the microstructure of ZnO quantum dots (QDs) in the SiO2 matrix. When Zn/Si <= 0.5, unaggregated quantum dots as small as 2 nm were distributed preferentially inside SiO2 spheres. When Zn/Si >= 1.0, however, ZnO QDs of similar to 7 nm were agglomerated and reached the SiO2 surface. When decreasing the ratio of the Zn/Si, a blue shift in the band gap of ZnO was observed from the UV/Visible absorption spectra, representing the quantum size effect. The photoluminescence emission spectra at room temperature denoted two wide peaks of deep-level defect-related emissions at 2.2-2.8 eV. When decreasing Zn/Si, the first peak at similar to 2.3 eV was blue-shifted in keeping with the decrease in the size of the QDs. Interestingly, the second visible peak at 2.8 eV disappeared in the surface-exposed ZnO QDs when Zn/Si >= 1.0.