The degradation of lightly doped drain (LDD) n‐channel MOSFET's has been investigated in detail making use of ultraviolet light irradiation, and the following results have been obtained. The degradation of the parasitic MOSFET consisting of the gate, the LDD region, and the insulator in between, which dominates the deterioration of the total performance, can be divided into two phases. In the initial phase, the degradation is rapid compared to that in the succeeding phase. The initial phase degradation is due not to surface‐state generation, but to electron capturing of the oxide covering the LDD region which has a large number of traps. The quality of the insulator above the LDD region should be carefully controlled in order to make highly reliable LDD MOSFET's.