The specimen (0.05%C mild steel) stress-aged at 100°C and 200°C for a suitable time was stretched at room temperature, and then strain-aged at a temperature range of 50∼200°. The amount of strain-age hardening of astress-aged specimen is much less than that of an unstress-aged one. The higher the aging-temperature and the longer the aging-time are the more remarkable is this tendency. When the stress-aging is tried on specimens for 6 min at 200°C twice in succession, they show only little strain-age hardening and recovery of the yeild-point. From these results, it will be found that a non-ageable mild steel may be obtained by stress-aging under suitable condition. The results can be explained as follows. In the process of stress-aging, not only a locking of dislocations by Cottrell’s effect but also a precipittation of excesive solute atoms at the dislocation must take place so that the amount of solute atoms in ferrite, which contribute to stress-aging of steel, decrease extremely. In the light of the above investigation, stress-aging may be utilized for preventing the stretcher-strain of sheet steels for deep drawing.