In this paper, a facile and green method was employed to synthesize CdSe quantum dots (QDs)-modified TiO2. It exhibited much higher photocatalytic activities than both TiO2 and bulk CdSe-modified TiO2 on degrading malachite green (MG) aqueous solution under visible light irradiation. The vital role that quantum size effect of CdSe QDs plays in enhancing photocatalytic properties of CdSe QDs-modified TiO2 is discussed in detail for the first time. Due to the more negative conduction band potentials of CdSe QDs than that of bulk CdSe, the electron transfer rate from CdSe QDs to TiO2 is much faster than that from bulk CdSe to TiO2, which would be beneficial for an efficient charge carrier's separation. As a result, more quantities of active species (O2 center dot(-), H2O2,center dot OH) are generated during the photocataltytic reaction in CdSe QDs-modified TiO2 system. This work could not only provide a facile and green strategy to fabricate semiconductor QDs-modified TiO2 photocatalyst but also contribute to a deeper understanding of the specific role of semiconductor QDs during the photocatalytic reaction process. (C) 2014 Elsevier B.V. All rights reserved.