Two new degradation products for Anastrozole active pharmaceutical ingredient (ANZ) have been identified and reported in this paper. The ANZ was subjected to thermal, photolytic, oxidative and base stress conditions prescribed by ICH guidelines. Separation of ANZ from its existing impurities and the two new impurities was achieved by using on Oyster ODS-3 (100 mm × 4.6 mm × 3.0 μm) column with an isocratic mixture of 10 mM ammonium formate and acetonitrile in the ratio 60:40 (v/v). The flow rate was 0.5 ml min−1. The elution was monitored at 215 nm. An isocratic stability indicating reverse phase liquid chromatographic (RP-LC) and LC–MS/MS method was developed for the determination of purity and assay of ANZ through forced degradation studies. The two new impurities detected were further subjected to spectroscopic studies. Based on the results obtained from the different spectroscopic studies, these impurities have been characterized as 2,2′-(5-((1H-1,2,4-triazol-1-yl)methyl)-1,3-phenylene)bis(2-methylpropanoicacid) (Diacid) and 2-(3-((1H-1,2,4-triazol-1-yl)methyl)-5-(2-cyanopropan-2-yl)phenyl)-2-methylpropanoicacid (Monoacid). ANZ was found to degrade in base, slightly in oxidative degradation conditions. The degradation products were well resolved from main peak and its impurities thus proved the stability, indicating power of the method. The developed method was validated as per International Conference on Harmonization (ICH) guidelines with respect to specificity, limit of detection, limit of quantitation, precision, linearity, accuracy, robustness and system suitability.