Opuntia ficus-indica, commonly known as prickly pear cactus, is commercially grown as a food and medicinal plant in Jeju Island, Korea. The crushed pads and fruits of Opuntia ficus-indica were dried in a freeze-dryer and ground into flour to be used for food materials. The major components of proximate compositions were part of a nitrogen free extract. The major minerals were Ca (4391.2-2086.9㎎%), K (1932.1-2608.7㎎%), and Mg (800.6-1984.8㎎%). The major amino acid was glutamic acid, comprising 16.3% of total amino acids in fruit and 25.2% in pad. Dihydroflavonols were identified as (+)-trans-dihydrokaempferol and (+)-trans-dihydroquercetin. Citric acid methyl esters extracted from fruits showed inhibitory activities against monoamine oxidase-B. The presence of trimethyl citrate has been reported in other plants, but 1,3-dimethyl citrate and 1-monomethyl citrate have not been previously reported. The results of pharmacological efficacy tests, including serum biochemical and hematological parameters, autonomic nervous system, anti-inflammatory, analgestic activity, anti-diabetic activity, antithrombotic, anticoagulant, dopamine beta-hydroxylase, monoamine oxidase activity, hyperlipidemia, the respiratory system, antigastic, and anti-ulcerative actions indicate that the fruit and pad of the Opuntia ficus-indica are rich sources of food and medicinal materials.