Chemically modified electrodes with Methylene Green (MG) incorporated into carbon paste exhibit excellent electrocatalytic ability for reduced nicotinamide adenine dinucleotide (NADH), reducing oxidation overpotential by 600mV compared with the direct oxidation of NADH at unmodified electrode. In flow injection analysis (FIA) of NADH, MG-modified carbon paste electrodes show high catalytic activity and stability, with linear response range over about 3 orders of magnitude and a detection limit of 1.0 pmol. Furthermore, the influence of various experimental conditions was explored for optimum analytical performance.