This paper presents a new and fast method for determination of vitamin B1 (thiamine) using functionalized CdSe/ZnS quantum dots (QDs) as a sensor. In this study the core-shell CdSe/ZnS QDs with thioglycolic acid capping agents were synthesized by wet chemical method in aqueous solution. Electronic absorption was applied for size determination of the QDs, the average diameters of the synthesized quantum dots were calculated using wavelength of the first excitonic absorption peak. The methods of FESEM and FTIR were also used for characterization of the prepared samples. The results indicated that vitamin B1 could effectively quench the QDs emission and the fluorescence intensity decreased linearly with the concentration of vitamin B1 ranging from 10-7 to 10-4 molL−1 with limit of detection of 2×10-8 mol L−1. Determination of vitamin B1in pills from two pharmaceutical companies were used as real samples using this method .