Filter media for respirator applications are typically exposed to cyclic flow condition instead of constant flow adopted in the standard filter media tests. The objective of this study is thus to investigate the effects of breathing frequency (BF) and peak inhalation flow rate (PIFR) on the peak particle penetration of respirator filter media, especially for particles at the most penetration particle sizes under the constant flow condition having equivalent mean inhalation flowrate (MIFR). Five respirator filter media were evaluated under inhalation-only conditions. Three BFs and three PIFRs were selected for the testing. Our study evidenced that both BF and PIFR would increase the peak particle penetration under the cyclic inhalation-only conditions. It is further found that, for each filter media, the peak particle penetration at various PIFRs could be merged into one curve via the newly-defined peak penetration ratio (i.e., the ratio of peak particle penetration at cyclic flow condition to the penetration at the constant flow condition having the equivalent MIFR) and the curve is only a function of BF. The above observation indicates that the increase of peak particle penetration resulted from the increase of PIFR is simply because of the increase of MIFR. The effect of BF on the peak particle penetration is clearly observed using the defined penetration ratio. Based on our finding a semi-theoretical model was further proposed to estimate the peak particle penetration of respirator filter media under inhalation-only conditions.