A novel nitrogen-doped carbon quantum dots (N-CDs) were prepared by the microwave irradiation method. The fluorescence quenching effect of Co(II) on the N-CDs was studied in the sodium dodecyl sulfate (SDS) medium and the fluorescence quenching was sensitized in the SDS. The linear range of calibration curve for the determination of Co(II) was 0.17 μg/mL–11.8 μg/mL and the limit of detection was 0.052 μg/mL. The method has been applied for the determination of Co(II) in samples with satisfactory results.