Cassia is a large tropical genus with about 600 species that have been widely used as folk medicines in China and India. This genus has been known to possess various biological activities, e.g. antimicrobial, anti-inflammatory, antioxidant, anti-malarial, anti-mutagenic activity, and anti-fertility etc. Flavonoids, for its broad spectrum of pharmacological activity and low toxicity, have attracted more interest in the development and utilization of natural medicines. The structure and biological activity research of flavonoids extracted from Cassia genus is the first step in the search for new drugs from those plants. This review summarizes the isolation and characterization of flavonoids from various Cassia species, such as Cassia absus, Cassia alata, Cassia fistula, etc. Flavonoids can be extracted from different parts of the plants, such as seed, leaf, stem and pod. Chemical structure research of these flavonoids in extracts has revealed many different types of compounds, which show the complication of the metabolism of Cassia genus. The antidiabetic activities can be found in many Cassia species. The efficiency of extraction method and action mode have been widely investigated. The extract not only can reduce the blood glucose level, but also improve glycogen content. Research show that the methanolic extract is effective in inducing hypoglycemic effects in both type I and II diabetes. Because flavonoids have complex structures, various function points, and unknown pertinence and selectivity for different health conditions, there are still many research areas waiting to be explored, such as to reveal the metabolic pathways of flavonoids in the Cassia genus, and to illustrate the structure-activity relationship between flavonoids and protein. That above-mentioned research will provide the basis for further medicinal development on this genus.