A novel core/shell organic nanoparticles, anthracene/poly-acrylamide (AN/PAM), has been prepared successfully. Based on the fluorescence quenching of AN/PAM nanoparticles by Cr(VI), a method for the selective determination of Cr(VI), without separation of Cr(VI) in water, was developed. Furthermore, the reaction mechanism between nano-AN/PAM and Cr(VI) was also discussed. The synthesis and reaction conditions were investigated in detail. The assay is characterized by short reaction time, very few interference stable fluorescence signals, simple instruments and sensitivity. Under optical experimental conditions, a limit of detection of 0.02μg/ml was achieved. The calibration curve was linear over the concentration range 0.04–2.00μg/ml with a correlation coefficient of 0.9924. The proposed method has been applied to the selective quantification of Cr(VI) in synthetic samples and waste-water samples with the satisfactory results.