The molecular dynamics method was used to simulate the shear along the <111> direction in Hadfield steel and a pure fcc Fe crystal. The stress-strain curves are obtained depending on the shear rate, the size of the computational cell, and temperature. It is shown that the shear rate in the range of 10–100 m/s has little effect on the theoretical strength at a constant temperature. With increasing temperature, the slope of the stress-strain dependences in the elastic region decreased, which is due to the temperature dependence of the elastic moduli. In addition, the temperature significantly influenced the theoretical strength – with an increase in temperature, plastic deformation began in ideal crystals at lower deformation values. Moreover, this dependence was more pronounced for a pure fcc Fe crystal than for Hadfield steel, which initially had structural imperfections caused by the presence of impurities that facilitate the initiation of plastic shears in a pure crystal. In this regard, at medium and low temperatures, the theoretical strength of pure iron was higher than that of steel. But at high temperatures (above 1200 K), its values for both materials became almost the same.