Flash sintering is a sintering technology coupled with temperature field and electric field, with characteristics of rapid mass transfer at low temperature, showing significant advantages in the synthesis of high entropy ceramics. In this study, relatively dense high entropy oxide ceramic (MgCoNiCuZn)O was synthesized by flash sintering, which properties were compared with those of conventional sintered samples. Under flash sintering condition of room temperature, the electric field intensity of 50 V/cm and the current density of 300 mA/mm², the time of phase transformation is only 10 s. The maximum relative density of flash sintered sample is 94%, which is 22.8% higher than that of conventional sintered sample. The maximum hardness of flash sintered sample is 5.05 GPa, which is 3.95 GPa higher than that of conventional sintered sample. When the frequency is lower than 2 Hz, the dielectric constant of flash sintering sample is one order of magnitude higher than that of conventional sintered sample. The property improvement of flash sintered samples is attributed to the acceleration of mass transfer by the critical electric field to increase the material density, and the extra defects introduced by the critical electric field.