The study on the chemical composition of the leaves of one of the most popularly known tropical plants, Senna siamea (Cassia leaves) has been carried out by analyzing samples of the plant leaves collected from Ado-Ekiti in Ekiti State for chemical composition. The proximate, elemental, phytochemicals and toxicant composition of the leaves of senna siamea were determined by analyzing samples of identified leaves using recommended method of analysis. The result of the analysis shows that the percentage crude protein, crude fibre, moisture content, ash content, carbohydrate and crude fat of the leaves are 4.01%, 12.36%, 46.01%, 17.93%, 7.67% and 12.02% respectively. The result of the mineral composition in PPM (Part per million) shows that iron, magnesium, manganese, potassium, calcium, sodium, copper, phosphorus and lead are 112.00, 876.00, 35.10, 812.00, 932.00, 612.00, 0.84 and 0.34 respectively while cadmium and vanadium was not detected in the leaves. The photochemical analysis shows that the leaves contain anthraquinones, alkaloids, phylobatannins and saponin and the toxicant composition shows the presence of tannin, oxalate and phytate. The minerals present in leaves shows that the leaves are good sources of essential nutrients but the presence of the toxicants shows that these leaves should be properly processed before consuming them.