A silanization technique of hydrophobic quantum dots (QDs) was applied to SiO₂-coated CdSe/Cd_xZn_1–xS QDs to precisely control the SiO₂ shell thickness and retain the original high photoluminescence (PL) properties of the QDs. Hydrophobic CdSe/Cd_xZn_1–xS core–shell QDs with PL peak wavelengths of 600 and 652 nm were prepared by a facile organic route by using oleic acid (OA) as a capping agent. The QDs were silanized by using partially hydrolyzed tetraethyl orthosilicate by replacing surface OA. These silanized QDs were subsequently encapsulated in a SiO₂ shell by a reverse micelles synthesis. The silanization plays an important role for the QDs to be coated with a homogeneous SiO₂ shell and retain a high PL efficiency in water. Transmission electron microscopy observation shows that the shells are 1–9 nm with final particle sizes of 10–25 nm, depending on the initial QD size. In the case of short reaction time (6 h), the QDs were coated with a very thin SiO₂ layer because no visible SiO₂ shell was observed but transferred into the water phase. The silica coating does not change the PL peak wavelength of the QDs. The full width at half-maximum of PL was decreased 4 nm after coating for QDs emitting at both 600 and 652 nm. The PL efficiency of the SiO₂-coated is up to 40%, mainly determined by the initial PL efficiency of the underlying CdSe/Cd_xZn_1–xS QDs.