between two-level atoms and cavity is strong enough, the spontaneously emitted photons may be repeatedly absorbed and emitted by the atoms before they leave the cavity and the corresponding spectrum exhibits two peaks termed as vacuum Rabi splitting [8,9], which has been studied in atomic beams [10], in cold atomic cloud [11], and even in hot atomic vapor [12]. If a coherently prepared three-level atomic medium is placed inside the optical cavity under the strong-coupling condition, the "dark-state polariton" peak [13][14][15][16] appears in the cavity transmission spectrum (CTS) due to the intra-cavity electromagnetically induced transparency (EIT). It is worth mentioning that the main concern in the above investigations is the atom-cavity coupling. Recently, vacuum-induced transparency [17] has been reported by using an ensemble of cold atoms strongly coupled to an optical cavity, where the control light can be generated from the electromagnetic vacuum field assisted by the atom-cavity coupling.In addition, phase-conjugate four-wave mixing (FWM) and six-wave mixing (SWM) processes based on thirdorder and fifth-order nonlinear processes in coherent atomic media [18][19][20] have also attracted lots of attention in recent years, and the coexistence of these two nonlinear processes due to double EIT windows and atomic coherences has been reported [21,22]. We put the coexisting FWM and SWM processes into the cavity and report the bright-state polaritons of FWM and SWM signals and their coexisting cavity modes [23].In this letter, we place the phase-conjugate FWM process inside an optical cavity. We investigate the control effect of atom-cavity coupling in the FWM process. We have observed the vacuum-induced suppression and enhancement of FWM signal due to the atom-cavity coupling. We explain the vacuum-induced suppression and enhancement by using the dressed-state picture, in which Abstract We report on an experimental study of vacuuminduced suppression and enhancement of four-wave mixing (FWM) signal in a composite atom-cavity system. By scanning the additional dressing field, the suppression ratio of the FWM signal can reach 90 % compared with 40 % without cavity. We attribute the enhanced suppression and enhancement to the atom-cavity coupling arising from a vacuum-induced Raman process, which amplifies the dressing effect from the additional field. Also, the dressing asymmetry of the atom-cavity coupling is discussed and used to estimate the nonlinearity of atomic medium in the cavity. The suppression and enhancement can be interpreted by a dressed-state picture and agree with theoretical calculations. The investigation may find applications in optical switch and quantum memory controlled by cavity.