An air-stable, low-toxicity, single-molecular source for ZnS is dernonstrated to be an appropriate reagent to synthesize highly luminescent ZnS-capped CdSe with a narrow size distribution. A photoluminescence quantum yield of above 50 % and a photoluminescence peak full width at half maximum of around 32 nm could be obtained after synthesis using a microreactor. The surface of the ZnS-capped CdSe nanocrystals can be hydrophilic, while retaining the high quantum yield. Microscopic observation shows that accurate time control, which could be achieved by using a microreactor, is important to avoid the formation of many isolated ZnS particles and the deterioration of the luminescence.