Spontaneous vitrification was observed in the equilibrium immiscible Fe‐Cu system. The metastable phase before vitrification was an icosahedral incommensurate phase which was formed by an ion beam mixing of Fe‐Cu multilayer films at room temperature and subsequent high‐temperature thermal annealing. The electrical and magnetic properties of the icosahedral phase are also reported, and the relation with spontaneous vitrification is discussed.