The objective of this investigation is to deepen the understanding of microwave effect on the reactive sintering of ceramics powders. The formation of the spinel ZnAl2O4 from Al2O3 and ZnO powders have been studied during conventional and 2.45 GHz microwave-assisted heating processes under the same thermal cycle. The continuous dilatometric curves under both heating processes are presented. The experimental results showed that electromagnetic field had significant influence on reactive sintering: the solid-phase reaction took place at a lower temperature and at a higher rate under microwave heating. Furthermore, the difference in response to electromagnetic field between alumina and ZnO made a different microstructural evolution compared to conventional reactive sintering process.