S-Adenosylmethionine (SAM) serves as the major methyl group donor in biological system. This study describes an upgrade method for radiolabelled biosynthesis of SAM by methionine adenosyltransferase (MAT) in the presence of ATP and L-[^(14)C-methyl]methionine. The MAT was partially purified from rat liver and reoptimized the assay condition. The final yield of biosynthesis was better than 90% with relatively high specific radioactivity.