A 6,7-dihydroxy-3-methyl-9-thia-4,4a-diazafluoren-2-one/multi-wall carbon nanotubes (DDF/MWCNT) film-modified carbon paste electrode (CPE) has been fabricated. The electrochemical oxidation of the behavior of reduced nicotinamide adenine dinucleotide (NADH) has been studied at DDF/MWCNT film-modified CPE by electrochemical methods. The DDF/MWCNT film-modified CPE showed a pair of well-defined, quasi-reversible cyclic voltammogram (CV) which corresponds to the transformation of catechol to o-benzoquinone redox couple. With this modified electrode the oxidation potential of the NADH was shifted about 0.333 V toward a less positive value, presenting a peak current much higher than those measured on an unmodified CPE. Detailed analysis of cyclic voltammograms and chronoamperogram gave fundamental electrochemical parameters including the electroactive surface coverage (Gamma), the transfer coefficient (alpha) and the catalytic rate constant (k'). Under the selected conditions, two linear calibration curves for NADH were obtained in the concentration ranges of 1-10 nM and 10-80 nM with the detection limit of 0.7 nM. This modified electrode was quite effective to detect the trace amounts of NADH in the presence of the ascorbic acid (AA) or uric acid (UA). (C) 2009 Published by Elsevier B.V.