The main purposes of this reseach are to find out some conditions in which bending cracks initiate and propagate and to propose a new reasonable and practical method of bending test. The behaviour of wide thick steel plate under uniform bending of large displacement and in large plastic strain were calculated as plane strain state with a new displacement function θ and with plastic strain increment Δεθ=ln√ B - ( AB / r ) ². Using the Tresca's strain hardening law and stress-strain diagrams of tensile test and compressive test, or tortional test, the behaviour of bending test was analysed by the method with good accuracy. In order to find some practical bending test method, bending test under concentrated load were carried out. Relations between bending moment and bending strain from the test agreed with theoretical values, if the frictional force at supported ends were considered. Effects of this frictional force on the magnitude of bending moment of specimen are pretty large, but the effect on the initiation and propagation of bending cracks are very small. For these tests, the effects of the shape of specimen, radius of mandrel and critical bending angle were found and some reasonable method of bending test for thick wild steel was proposed.