AbstractWe have investigated the active‐layer‐thickness dependence of Rabi splitting energies in ZnO microcavities. We fabricated ZnO microcavities using rf magnetron sputtering for the HfO2/SiO2 distributed Bragg reflector and pulsed‐laser deposition for the ZnO active layer. In order to control of the Rabi splitting energies, the active layer thickness was changed from λ/2 to 3λ/2. In angle‐resolved reflectance spectra at 10 K, the cavity polaritons resulting from the strong coupling between A, B, and C excitons peculiar to ZnO and the cavity photon were clearly detected. We estimated the energies of the exciton‐photon interaction, the so‐called Rabi splitting energies, from the analysis of the cavity polariton dispersions using the phenomenological Hamiltonian for the strong exciton‐photon coupling. The Rabi splitting energies markedly increase with an increase in the active layer thickness. The active‐layer‐thickness dependence of the Rabi splitting energies are explained by a semi‐quantitatively analysis. Consequently, we have succeeded in the control of the Rabi splitting energies in the ZnO microcavities. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)