Charpy impact test is explained from the viewpoint of the transition of hammer's energy to the kinetic energy of dislocations. Ductile-brittle transition temperature is &theta; is represented as a simple function of the nucleation energy of dislocations ε_n, the dislocation density ρ, the maximum density of mobile dislocations ρ₀ and the interaction energy of obstacles with dislocations U_ob; when ρ<ρ₀, while when ρ>ρ₀, where k is Boltzmann constant and ρ_m is the density of mobile dislocations. The relationship is compared with the experimental results for cold rolled S15C steels and neutron irradiated low and high copper A533B steels of nuclear reactor pressure vessel materials. The experimental results are qualitatively explained by the present model.