Glycosylation, one of the most common and important reactions in biological systems, results in diverse functions and is often found in biologically active small-molecule natural products produced by microorganisms.  Furthermore, sugar moieties are generally critical for their activities.  Alternating the sugar structures thus provides the potentials for enhancing the biological activities of natural products, which evokes researchers to study the sugar biosynthetic machinery and its application in the modification of sugar moieties with an aim of generating unnaturally glycosylated natural product drugs with better activities.  This review will briefly outline current studies on sugar biosynthesis and glycosyltransferase, with a few selected experiments designed to alter natural-product sugar structures.