If a rectangular beam is heated such that the temperature is uniform in the upper fraction keeping the rest unheated and then wholly cooled to the room temperature, the residual bending deformation is occured plastically. The author analized the phenomena as one of the fundamental problems of thermo-plastic working, which is distinguished from the usual plastic working. In theoretical study, the following assumptions are used : 1) The elastic constants and thermal expansion coefficients do not change during the process. 2) The decrease of yield stress is proportional to the increase of temperature. 3) We use Prandtl-Reuss stress-strain equations and von-Mises yield criterion as the plastic laws of material. 4) The depth of heated region is less than one half of the height of beam, and the temperature changes discontinuously at the boundary of heated zone. 5) Navier's hypothesis is applied to the bending deflection of beam. Under these assumptions, the characteristics of plastic behaviours of beam are studied. The numerical calculations are performed for initially unstrained beam and the results are compared with the experimental deta with fair agreement.