A specific TE10m microwave cavity has been designed to follow-up the shrinkage during the microwave sintering of ceramics powders using an optical based position sensing device. The basic principle consists in measuring the distance from a laser source to the sample surface by means of a triangulation method. The spatial resolution device is around a few micrometers that enables to accurately measure the shrinkage versus time of a microwave irradiated sample. The shrinkage curves have been recorded during the direct microwave sintering of CuO and ZnO. Sintering kinetics has been found extraordinarily fast as only a few seconds are needed to achieve the maximum shrinkage for both materials. This new method is undoubtedly powerful to increase our understanding of microwave sintering and very useful to control the microstructure of microwave sintered ceramics.