In order to investigate the conditions for temperature and stress which govern the propagation of brittle crack in mild steels, the authors performed special double tension tests using specimens made of 12mm thick rimmed steel. The specimens were composed of two parts, initial crack was started by statical tension in small attached part so as to reach at the edge of main part, where this initial brittle crack was tested whether it would propagate through the test piece or not under various combinations of tensile stress and temperature. In the other part, the mechanism of propagation of brittle crack through mild steel plates was discussed. Namely, assuming a slip formed at the root of a notch by concentration of stress may finally develope to a small crack and calculating the variation of stored energy in the plate during this change, it was found that a brittle crack once started will propagate indefinitely or in other word the energy stored in cracked plate may diminish with the increase of crack length at low temperature and high stress, while at hihger temperature and lower stress there is a minimum energy at certain crack length and the initial crack will stop at this length. At certain temperature and stress, the energy curve may have a inflexion point with horizontal tangent at a definite crack length, and under this condition the critical stress and temperature for propagation of brittle crack may be defined. The relation thus obtained between the critical stress, critical length and temperature were compared with experimental results and it was found that there were good coincidence between these two.