Abstact: A simple , accurate and sensitive spectrophotometric method has been developed for the determination of amoxicillin trihydrate (AMOX) as pure and in pharmaceutical preparations (capsule) . This method based on the coupling reaction of drug with diazotized 2,4- dinitroaniline reagent in an alkaline medium at 20 ºC to produce an intense red, water-soluble dye that is stable and has a maximum absorption at 555 nm . Beer's law was obeyed over the concentration range 3-16 µg/ml with molar absorptivity of 1.1 x104 l.mol-1.cm-1. The limit of detection(LOD) is 0.1448 µg/ml while the limit of quantitation(LOQ) is 0.4825 µg/ml. The method shows high accuracy (average recovery 100.43%) and precision(relative standard division (RSD) is less than 1.4%).The stoichiometry of the resulting azo dye has been also worked out and it is found to be 1:1 AMOX:Diazotized 2,4-Dinitroaniline.Standard addition method refers to the suggested method is free from interferences from common excipients. The developed method was successfully applied for the determination of studied drug in capsules comparable with the certified content value. .