Monodisperse cadmium sulphide (CdS) quantum dots (QDs) with a tunable size from 1.4 to 4.3 nm were synthesized by a non-injection method, and their surface states were characterized by photoluminescence spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The steady state photoluminescence study identified that the proportion of the trap state emission increased with the QD size decrease, while from the photoluminescence decay study, it appeared that the trap state emission results from the emission via a surface deep trap state. The XPS measurements revealed the existence of surface Cd with sulfur vacancy sites which act as electron trap sites, and the population of these sites increases with the QD size decrease. These results are consistent to conclude that the trap state emission mainly originates from the surface deep trapped electrons at the surface Cd with sulfur vacancy sites.