An emulsion liquid membrane containing Span-80, CHCl3 and o-phenanthroline, as a mobile carrier is proposed for the preparation of nanosized semiconductor CdS particles. Effects of the different factors on the synthetic reaction were investigated. The structure and morphology of the products were characterized, and the opto-electronic properties were detected. Under the conditions of Roi 2.0, Rew 1.5 and pH 5, in 10 min of contact between ELM and the external aqueous phase, the CdCl2 in the external aqueous phase was completely transferred and reacted with the Na2S in internal aqueous phase, and turned into size-quantized CdS nanocrystals with the average size of about 4.5 nm. The ELM method may open a new synthetic route for a series of nanostructured inorganic materials.