Trace amounts of nitric oxide (NO) have been determined in aqueous phosphate buffer solutions (pH=7.4) by using a glassy carbon electrode coated with three charge-different polymer films. The glassy carbon electrode was coated first with negatively charged Nafion film containing tetrakis(pentafluorophenylporphyrin) iron(III) chloride (Fe(III)TPFPP) as the NO oxidation catalyst, and then with positively charged poly(acrylamide-co-diallyldimethylammonium chloride) (PADDA) and with neutral poly(dimethylsiloxane) (silicone) at the outermost layer. This polymer-coated electrode showed an excellent selectivity towards NO against possible concomitants in blood such as nitrite, ascorbic acid, uric acid, and dopamine. All current ratios between each concomitant and NO at the cyclic voltammogram was in 10 -3 ~ 10 -4 . This type of electrode showed a detection limit of 80 nM for NO. It was speculated from the electrochemical study in methanol that high-valent oxoiron(IV) of Fe(TPFPP) participated in the catalytic oxidation of NO.