Irradiation embrittlement in a low dose region was studied for an aluminum-killed carbon steel from the point of dose rate effects. The dose rate as low as ∼5x10⁸ n/cm²/s (E> 1MeV) caused a Charpy transition temperature shift of more than 30°C after irradiation to ∼10¹⁷n/cm² (E>1MeV) at 240°C, while a similar amount of Charpy shift was observed after ∼10¹⁸n/cm² (E >1MeV) at higher dose rate (∼7x10¹¹n/cm²/s). The effect of dose rate was obvious in the annealing response on hardness. The recovery characteristics indicated that more stable defects were produced in the material irradiated at a lower dose rate.