It has been said that material has proportional limit, elastic limit and yield point. But elastic limit is the beginning of molecular process in faults of lattice, and in technical experiment the measurement of such elastic limit is impossible and senseless. In precise technical experiment mild steel has yield point alone. Velocity of deformation of material is different from pulling speed and between elastic range and plastic. Since velocity of deformation increases at the beginning of slip, we can measure yield point by measureing the point at which velocity of deformation begins to increas. This can be measured by means of mirror type extensometer and the others. Therefore, we should adopt yield point alone and establish as follows ; "As yield point, we dicide the limit of stress, at which the first slip arises in material, or velocity of deformation of material begins to increase, in tensile test of definite constant elastic strain rate."