We have tested a series of high-Q photonic band gap (PBG) resonant cavities in the mm-wave regime and achieved a cavity-Q of 2.3×104, the highest value reported among all two- and three-dimensional PBG cavities. We have also systematically varied the size and reflectivity of such cavities to study their effect on cavity properties such as cavity modal frequency, linewidth, and cavity Q value. We show that the resonant frequencies can be tuned throughout the PBG regime and that linewidths (or equivalently Q value) can be varied over two orders of magnitude (i.e., a Q value from ∼2.7×102 to 2.3×104).