Feruloyl-CoA:arabinoxylan-trisaccharide O-hydroxycinnamoyl transferase, which catalyzes the transfer of ferulic acid from Fer-CoA to arabinoxylan-trisaccharide in the formation of feruloyl arabinoxylan-trisaccharide (Fer-AXX), has been found in an ionically bound fraction and a cytosol fraction of suspension-cultured rice (Oriza sativa L. cv. Nipponbare) cells. Analysis of reaction products by high-performance liquid chromatography showed the formation of product A, which is one of the transfer products having the same retention time as authentic Fer-AXX. Product A was purified by reverse-phase chromatographies to characterize its structure. The isolated product A showed the same ultraviolet spectrum and molecular weight on fast atom bombardment mass spectrometric analysis as those of authentic Fer-AXX. Alakaline saponification of product A released ferulic acid and oligosaccharide. The released oligosaccharide consisted of arabinose and xylose in a molar ratio of 1:2. These results support the identity of product A as feruloylated arabinoxylan-trisaccharide and show the existence of a feruloyltransferase catalyzing the feruloylation of a hemicellulosic fragment.