AbstractThis paper discusses electroluminescent (EL) characteristics of ZnS : Mn orange‐yellow EL cells. By measuring charage density within the EL cell by means of the Sawyer‐Tower circuit, the dependence of EL characteristics on insulating layer thickness and active layer thickness was determined. Clamp voltage of EL cells estimated from the bend point in the charge density‐voltage characteristic curve was in good agreement with the threshold voltage. It was found that below the clamp voltage, it was possible to treat active layer as an insulator. By decreasing the insulating layer thickness and thus increasing the insulating layer capacitance, we found the following: (1) the voltage of the insulating layer decreased and the clamp voltage decreased; (2) at the same time, the drive voltage margin defined by the difference between the breakdown voltage and the clamp voltage decreased. When the active layer thickness was decreased, the active layer voltage decreased and the insulating layer voltage increased. Furthermore, in the EL cell where the insulating layer capacitance per unit area was 10 nF/cm2, the clamp voltage became smallest when the active layer thickness was taken to be approximately equal to 0.3 μm. Based on these results, an experimental formula was obtained which describes the relation among the clamp voltage, the insulating layer capacitance, and the active layer thickness.