It is well known that large plastic deformation occurs during the phase transformation in a cooling process of the steel. Because this phenomenon (called a phase transformation plasticity) has great influence on mechanical behavior of the steel, it is important to formulate it in order to calculate the deformation and residual stresses precisely. To obtain a phase transformation plasticity coefficient, we developed a simple experimental method using a three-point bending test under constant cooling rate. From experiments, the relationship between transformation plasticity coefficient and cooling rate is determined and formulated. Also, residual stresses in a steel slab are numerically predicted using this formula.