Cassia species are medicinally useful and has innumerable traditional therapeutic applications. Natural antioxidants such as dietary plant phenolics and flavonoids are increasingly attracting attention. Cassia is one such most important plant which is a source of antioxidants i.e phenolics and flavonoids. Quantitative determination of phenols and flavonoids in the leaves of 12 species of Cassia was carried out using spectrophotometric methods. Gallic Acid and Quercetin reagents were used as standards for calibaration of phenols and flavonoids respectively. Comparative spectrogram of different Cassia species show that Cassia auriculata has the highest phenolic and Cassia serecea the highest flavonoid content.