The change of Young's modulus of elasticity by magnetization, ΔE/E, in annealed Armco iron and various carbon steels was measured by the method of magnetostrictive oscillation. Main results obtained are as follows:-Young's modulus of elasticity in these materials increases by magnetization, and the amount of increase is of the order of 10-3. The rate of increase is large in weak magnetic fields, and becomes gradually less as the field increases. For a given magnetic field, ΔE/E decreases continuously as the carbon content increases. Every ΔE/E vs. intensity of magnetization curve consists of three stages. Akulov and Kondorsky's theory was confirmed for the total change of elasticity in Armco iron. The initial susceptibility plays an important role in the change of Young's modulus of elasticity by magnetization in iron and carbon steels.