A simple and sensitive reverse phase ultra performance liquid chromatography (RP-UPLC) method has been developed, optimized and validated for the simultaneous determination of N-Hydroxybenzotriazole (HOBt), Cinchonidine and 1,3-Dicyclohexyl urea (DCU) contents at low levels in fosinopril sodium drug substance. Efficient chromatographic separation was achieved on Acquity UPLC HSS C18 column, 100 mm long with 2.1 mm i.d., 1.8 µm particle diameter, thermo stated at 30°C. Gradient elution involving binary mixture of potassium dihydrogen orthophosphate (0.01M, pH:3.0±0.05 with ortho-phosphoric acid) and acetonitrile at a flow rate of 0.10 mL min-1 has been used. The analytes were monitored by photodiode array (PDA) detector set at 205 nm. The drug substance was subjected to stress conditions of hydrolysis, oxidation, photolysis, thermal and humidity degradation. The method was validated for specificity, sensitivity, linearity, precision, accuracy and solution stability. The limit of detection (LOD) and limit of quantification (LOQ) for HOBt, Cinchonidine and DCU were in the range of 0.85-3.52 ppm and 2.57-10.67 ppm, respectively. The average recoveries for HOBt, Cinchonidine and DCU are in the range of 98.1% to 102.6%. The method can be used for the routine quality control analysis of fosinopril sodium drug substance.