Summary: A grooved circular cavity is designed for the millimeter wave measurements of dielectric substrates. The grooves are introduced to separate the degenerate TE01p and TM11p modes in circular cavities. A rigorous mode-matching method is used to investigate the influence of grooves on both the TE01p and TM11p modes. The dimensions of the grooves are determined from the numerical results. Comparative experiments of circular cavities with and without grooves validate the design method.