We report the sintering behavior of nanocrystalline zinc oxide under external AC electric field between 0 and 160 V/cm. In situ acquisition of density by means of laser dilatometry, evaluation of specimen temperature, real-time measurement of electric field and current help analyze this peculiar behavior. Field strength and blocking electrodes significantly affect densification and microstructure, which was evaluated in the vicinity of the flash event and for the fully sintered material. High current densities flow through the sample at high electric fields, entailing a sudden increment of the temperature estimated to several hundreds of K and an exaggerated grain growth. In contrast, low current density flows through the sample at lower electric fields, which guarantees normal grain growth and highest final density. Macroscopic photoluminescence measurements give insights into the development of the defect structure. Electric fields are expected to enhance defect mobility, explaining the high densification rates observed during the sintering process.