Nowadays, one of the most important problems in the field of the strength of materials is the yielding condition under non-uniform stress distribution. This paper deals with the result of investigation on the yielding of bars under bending moment. The stress existing in the yielding baran is measured by means of X-ray and then the yielding mechanism is treated theoretically. When a bar is loaded in bending under an increasing load, the bending moment increases to some extent proportionally to the deflection, as is well known, even after the former excees the moment Ms, which is the moment value when the stress of the outermost layer just reaches the yield stress σs. That is, the yield stress seems to be raised. As the result of present experiment, however, stress value exceeding the definite yield stress σs is never found even at a moment over Ms. The measured stress distribution inside a bent bar subjected to a moment over Ms will be stated. Basing on the experimental results, the yielding mechanism under bending moment is theoretically treated. By the analysis the yielding phenomenon is found to be reasonably explained.