Two novel formazan derivatives, FNB and FDNB, were synthesized from the corresponding phenylhydrazones and characterized by various spectroscopic methods. The sensing ability of these formazan sensors towards different anions was determined in THF by a colorimetric UV-vis spectroscopy method. FNB had an efficient colorimetric sensing ability for fluoride anions by changing colour from red to green then blue with a Delta lambda shift of ca. 138 nm, whilst FDNB acted as a colorimetric sensor by changing colour from yellow to red with a Delta lambda shift of ca. 95 nm, which could be recognised by the naked eye. The detection limit of sensor FNB was found to be 0.051 ppm (under the WHO permitted level) in the presence of F- anions. Moreover, the binding forms of F- with the sensors were examined by H-1 NMR experiments. These results indicated that sensors FNB and FDNB, containing nitro groups and hydrazo unit (-NH) hydrogen bond donation, exhibited high sensitivity and selectivity toward fluoride, which has biological importance.