It is difficult to obtain γ-Fe2O3 nanostructures by heating iron substrate in ambient conditions because γ-Fe2O3 is less thermodynamically stable than α-Fe2O3. In this work, we synthesize γ-Fe2O3 nanowires by heating iron particles under an external force. The stacking style of iron and oxygen ions under a strong shearing stress tends to adopt the γ-Fe2O3 structure regardless of the thermodynamic restriction. These γ-Fe2O3 nanowires exhibit a clear ferromagnetic property. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurements confirm that γ-phase structure appears only under the applied external force during the heating period. A window of the magnitude of the external force is found to help the nanowire growth on iron particles. The growth mechanism of γ-Fe2O3 nanowires other than α-Fe2O3 under the external force is discussed and an applied stress-assisted growth model is proposed. This work presents an easy approach to produce ferromagnetic iron oxide nanowires on a large scale.