In the presence of cucurbit [7]uril (CB[7]), the CB[7] could react with palmatine, which served as a sensitive fluorescence probe, to form host-guest stable complexes and the fluorescence intensity of the complexes was greatly enhanced. The fluorescence intensity decreased linearly with an increasing number of L-cystine in the inclusion system. The experimental results show that there exists a competition between L-cystine and palmatine for the CB[7] hydrophobic cavity and L-cystine occupies the space of CB[7] cavity, leading palmatine molecules to be forced to reside in the aqueous environment. Based on the fluorescence quenching of the CB[7]/palmatine complexes resulting from complex formation between CB[7] and L-cystine, a spectrofluorimetric method for the determination of L-cystine in aqueous solution in the presence of CB [7] was developed. The linear relationship between the corresponding values of the fluorescence quenching โF and L-cystine concentration was obtained in the range of 6.0 to 1.5ร10 3 ngโขmL -1 , with a correlation coefficient (r) of 0.9996. The detection limit was 2.0 ngโขmL -1 . The application of the present method to the determination of L-cystine in tablets gave satisfactory results. This paper also discussed the mechanism of the fluorescence indicator probe.