Density gradient ultracentrifugation (DGUC) is widely used for physical isolation (enrichment rather than purification) of subcellular membrane vesicles. It has been a valuable tool to study specific subcellular localization and dynamic trafficking of proteins. While sucrose has been the main component of density gradients, a few years ago synthetic OptiPrep (iodixanol) began being used for separation of organelles because of its iso-osmotic property. Here, we describe a detailed protocol for density gradient fractionation of various mammalian subcellular vesicles, including endoplasmic reticulum (ER), Golgi apparatus, endosomes, and lipid rafts, as well as apical and basolateral membranes of polarized epithelial cells.