The thermal stress in an LED caused by junction heating has been calculated. It is shown theoretically that the change of light output with time is proportional to the third power of thermal stress in the junction. This result is experimentally verified. Mechanical‐stress experiments demonstrate that degradation may occur by means of mechanical stress alone. Using the two‐path model for the quantum efficiency and the time dependence of the stress‐induced lattice‐defect concentration, a degradation formula is derived which can be verified to fit the behavior of many LED’s. The characteristic tapering off seen in most degradation curves can be attributed to strain hardening (dislocation pinning). A dark‐line‐defect progression is observed.