A simple and green fluorescence detection system on the basis of the quenching effect of Cu(II) ions on the graphitic carbon nitride nanosheets (g-CN NS) fluorescence and recording the changes of restored fluorescence intensity upon the addition of metformin (MET) was developed for the first time. The g-CN NS were produced using high-temperature polymerization of melamine followed by ultrasonication-assisted liquid exfoliation. The method exhibited excellent sensitivity and accuracy toward MET with the detection as low as 3?nM and linear calibration curve in the range of 0.01-20??M. In addition, this assay was successfully applied to the determination of MET concentration in human serum samples. It is robust and is suitable for fast, sensitive and cost effective measurement of MET in biological materials.