Cycloaddition of various aryl isocyanates with oxiranes was realized in the presence of 50 mol % MgI2 etherate in a mild, efficient, and highly regioselective manner, which afforded 3-aryloxazolidin-2-one. Solvents (i.e., THF) and amounts of MgI2 etherate catalyst played the critical roles for the unique reactivity of this catalytic system.