Effect of metallurgical factors on the bake hardenability (BH) of low-C Al-killed steel is studied. The _yield stress change due to baking (ΔYS) is determined by grain size and dissolved (C+N) content. On the other hand, the ultimate tensile stress change due to baking (ΔTS) depends mostly on the dissolved (C+N) content. The locking mechanism of dislocation is discussed in relation to grain size dependence of these properties. The influence of alloying elements (Mn, Si, P) on BH through their metallurgical factors is also studied. Mn decreases ΔYS through the reduction of dissolved C content. Si gives rise to the increase of dissolved C content and grain refining up to 0.5% Si content. Over 0.5% Si content, the grain becomes coarser in turn. Consequently, ΔYS increases up to 0.5% Si content and levels of after that. Though the addition of 0.06% P does not affect the dissolved (C+N) content, it gives rise to a little increase of ΔYS through grain refining. Larger grain size dependence of ΔYS is observed at -5ppm of dissolved (C+N) content for 0.06% P containing steel.