During development of the peripheral nervous system in mammals, Schwann cells wrap their plasma membranes around neuronal axons, forming multiple myelin sheaths. A mature myelin sheath insulates axons and increases nerve conduction velocity while protecting nerve fibers from various stresses such as physical ones. Despite this functional importance, the molecular units that underlie dynamic morphological changes in formation of myelin sheaths are not sufficiently understood. Arf1 is a small guanosine triphosphate-binding protein that plays multiple roles in intracellular trafficking and related signaling, both of which are processes involved in cell morphogenesis. We demonstrate that the Arf1 guanine nucleotide exchange factor, brefeldin A-inhibited guanine nucleotide-exchange protein 1 (BIG1)/Arfgef1, and the effector Arf1 regulate the initiation of myelination of axons by Schwann cells. Schwann cell-specific BIG1 conditional knockout mice, which have been generated here, exhibit reduced myelin thickness and decreased localization of myelin protein zero in the myelin membrane, compared with their littermate controls. BIG1 knockout mouse nerves specifically decrease the amounts of Arf1 in the AP1 clathrin adaptor protein subunits but not the Arf1 binding to GGA1 (Golgi-localized, gamma-adaptin ear-containing, Arf-binding protein 1) transporting proteins. The amounts of Arf1 in the COPI coatomer protein subunits were comparable in the knockout mice and controls. Similar results in myelin thickness are observed in Arf1 conditional knockout mice, which have also been generated here. Thus, the BIG1 and Arf1 unit plays a key role in Schwann cell myelination, newly adding it to the list of molecular units controlling myelination.