A rhodamine B derivative containing 1,2,4-triazole as subunit was characterized as an "off on" type Cu2+ selective fluorescent probe. It exhibited high selectivity and sensitivity for Cu2+ in ethanol water solution (9:1, v:v, pH 7.0, 20 mM HEPES) and underwent ring opening. A prominent fluorescence enhancement at 570 nm was observed in the presence of Cu2+ with the change in the absorption spectrum, and a 1:1 metal ligand complex was formed. With the optimized experimental conditions, the probe exhibited a dynamic response range for Cu2+ from 8.0 x 10(-7) to 7.5 x 10(-6) M with a detection limit of 2.3 x 10(-7) M in ethanol water solution (9:1, v:v, pH 7.0, 20 mM HEPES). Its application in Cu2+ imaging in living cells was also studied.