In our previous papers, plastic behaviour of a thick steel plate under uniform bending in two cases of plain strain and plain stress state was analysed. And relations between bending moment and bending strain obtained by our theoretical analysis were compared with experimental ones. In this paper, in order to study the condition of bending crack initiation, tensile tests were carried out on specimens with various transverse constrains. Comparing results of these tests with bending tests, it was found that the maximum elastic strain energy per unit volume stored in specimen at fracture might be considered as one of the criterion of crack initiation and relation between bending strain and tensile strain at the crack initiation could be explained by using the above criterion.