Low density polyethylene (LDPE) is widely used as a dielectric insulation of power cable. It has already reported that an incorporation of MgO nanoparticles in LDPE matrix could provide dielectrics with new and improved properties under dc high field. However under ac high field, it has already reported that the effect of the addition of MgO nanoparticles was relatively small compared with that under dc high field. This paper describes the dissipation current waveform and the electroluminescence (EL) of LDPE/MgO nanocomposite material under the high field trapezoidal waveform application by using the simultaneous observation system. From the results of experiments, the EL image and the relation between the dissipation current and the EL under high field trapezoidal waveform application were introduced.