A new, Simple, sensitive and accurate spectrophotometric methods have been developed for the determination of sulfamethoxazole (SMZ) drug in pure and dosage forms. This method based on the reaction of sulfamethoxazole (SMZ) with 1,2-napthoquinone-4-sulphonic acid (NQS) to form Nalkylamono naphthoquinone by replacement of the sulphonate group of the naphthoquinone sulphonic acid by an amino group. The colored chromogen shows absorption maximum at 460 nm. The optimum conditions of condensation reaction forms were investigated by (1) univariable method, by optimizing the effect of experimental variables (different bases, reagent concentration, borax concentration and reaction time), (2) central composite design (CCD) including the effect of three experimental factors (reagent concentration, borax concentration, and reaction time). The linearity ranges of sulfamethoxazole are (5-50) µg.mL-1 at 460 nm with molar absorptivity (6.7878×104- 7.0918×104) L.mol-1.cm-1, Sandell's sensitivity index (0.3755- 0.3571) μg.cm-2 and detection limit of (0.3755– 0.3594) µg.mL-1 for each procedure respectively. The results showed there are no interferences of excipients on the determination of the drug. The proposed method has been successfully applied for the determination of sulfamethoxazole in pure and pharmaceutical preparations.