We have investigated the characteristics of exciton polaritons in ZnO microcavities with different active layer thicknesses. The microcavity was made from a bulk ZnO active layer and two distributed Bragg reflectors (DBRs) consisting of HfO2 and SiO2 layers. We adopted rf magnetron sputtering and pulsed laser deposition for the preparation of the DBR and ZnO active layer, respectively. Angle-resolved reflectance spectra demonstrate the formation of cavity polaritons. From the analysis using a phenomenological Hamiltonian for the coupling between the cavity photon and three kinds of excitons labeled A, B, and C peculiar to ZnO, the vacuum Rabi-splitting energies in the λ/2-microcavity are estimated to be 30, 71, and 84 meV for the A, B, and C excitons, respectively. Moreover, we indicate the potential to control the Rabi-splitting energy by changing the active layer thickness.