Abstract: In this paper , the comprehensive relationship among TNDT, A (A =ln(ε0 /ε) and D -1/2 are researched in theory and experiment in smooth tensile specimen of 38CrA steel. The following relations are obtained : the relation between the yield stress σYS and A is an exponential one, the relation between ductile brittle transtition temperature TNDT and grain size D-1/2 is linear, and the multiplier of TNDT and In(ε0 /ε) - D-1/2 is a constant of material. According to the formulae mentioned above, three dimension T-ln(ε0 / ε) - D-1/2 cold brittle fracture controlling diagram and fracture mechanism's diagram are established. They can be used not only in analysis, diagnosis, prediction , controls and prevention of cold brittle fracture , but also in design, choice of materials and safe evaluation of structural parts in low temperature.

Key words: steel, cold brittle fracture, controlling diagram, mechanism's diagram