The article analyses and explains the physical and mechanical causes which form necking phenomenon in accordance with stress concentration which is caused by the facts that the actual specimens are not identical in properties and not uniform in diameters and strain hardening which results from dislocation congestion. 本文根据实际试件并非均匀等直引起的应力集中与位错塞积引起的形变强化,分析说明了形成颈缩现象的物理力学原因。