A covalent immobilization procedure for the phenothiazine redox mediator thionin using functionalized graphite powder as the immobilization matrix was developed. A carbon paste electrode constructed from this modified graphite powder served as a novel sensor for nitrite determination. The method of covalent immobilization of the mediator proved to be effective in anchoring the water soluble mediator for electrochemical studies. The modified electrode exhibited electrocatalytic activity for the reduction of nitrite at lesser potential with high sensitivity. The amperometric sensor can measure nitrite in the linear range from 1.66 × 10-5 M to 3.13 × 10-3M with a correlation coefficient of 0.9966 and a detection limit of 1.17 × 10-5M. Remarkable advantages of this electrode are its ease of fabrication, surface renewability, reproducibility and reasonable stability. © 2015 Scientific Publishers. All Rights Reserved.