Based on our model and theory of nonlinear deformation for BCC binary interstitial alloy built by the statistical moment method, we perform numerical calculations for characteristic quantities of nonlinear deformation such as the density of deformation energy, the maximum real stress, the limit of elastic deformation together with the stress-strain curve for metal Fe and alloy FeSi with the BCC structure at temperature up to 1100K, pressures up to 10 GPa, and silicon concentrations up to 4%. The calculated results of Fe are compared with experiments and the calculated results of FeSi are our new predictions.