Thietanes are pharmaceutically important cores of some biological compounds and intermediates of organic synthesis. Various thietanes were prepared from thiiranes via ring expansion through a reaction with trimethyloxosulfonium iodide in the presence of sodium hydride. The reaction process is a nucleophilic ring-opening reaction of thiiranes with dimethyloxosulfonium methylide, generated from trimethyloxosulfonium iodide and sodium hydride, and subsequent intramolecular displacement (cyclization) of thiolates to the dimethyloxosulfonium moiety. The current method provides a new strategy for efficient preparation of thietanes from readily available thiiranes.