Plants have many useful substances – and one of them is flavonoids with other phenolic compounds. This is a very big group of biological active compounds. Flavonoids have been referred to as "nature's biological response modifiers" because of their ability to modify the body's reaction to other compounds such as allergens, viruses, and carcinogens. They show anti-allergic, anti-inflammatory, and anti-cancer activity. In addition, flavonoids act as powerful antioxidants, providing remarkable protection against oxidative and free radical damage. As a result, consumers and food manufacturers have become increasingly interested in flavonoids for their healthful properties, especially their potential beneficial role in the prevention of cancer and cardiovascular diseases. Phenolic acids are interesting of their protective role against oxidative damage diseases (coronary heart disease, stroke, and cancers). One of possible sources of flavonoids and phenolic acids – Canadian thick-trefoil (showy trefoil) – Desmodium canadense (L) DC. This plant is not researched as good as many other plants, but there are some works on it. In Lithuania, KUM, there were some research works on Desmodium canadense (L.) DC. There was performed analysis of grass, collected in different phase of vegetation. Also was researched influence of mineral fertilising to amounts of flavonoids and phenolic acids.