L-Histidine, a biologically important amino acid, has been modified onto the silver electrode through covalent bonding. The coenzyme dihydronicotinamide adenine dinucleotide (NAD(P)H) was electrocatalytically oxidized to NAD(P)+at a L-histidine modified silver electrode (His/Ag). The electrochemical processes at the His/Ag electrode were investigated by using UV-visible spectroelectrochemistry in detail. Amperometric determination of NAD(P)H, at a potential of 0.25V (SCE), gave a linear response in the range of 1 × 10−7∼2 × 10−5mol/L with a detection limit of 0.05 μmol/L. The His/Ag electrode has been used in a FIA system with satisfactory results. Meanwhile, other proteins and chemicals present in samples do not interfere in the assay.