This work investigated the performance of two mechanical glass-fiber and one electret filter media under the oscillating flow conditions (i.e., cyclic flows in this study). DMA-classified particles of 30, 50, 100 and 150 nm, and oscillating flow conditions with the Average Suction Flow Rates (ASFRs) of 6.4, 25.6 L/min and oscillating frequency of 6, 24, 40 min-1 were chosen for the experiments. It is found in general the particle penetration of test media increases with the increase of oscillating frequency. The above-observed penetration enhancement is found to be more pronounced for small particles (i.e., 30 and 50 nm) as compared to that for large particles (i.e., 100 and 150 nm). At the flow oscillating frequency of 24 and 40 min(-1), the particle penetration through filter media is also higher than it under the corresponding constant flow conditions. The increase of ASFR increases the particle penetration of filter media for all the selected particle sizes. The apparent single fiber efficiency for media A and B under cyclic flow conditions was lastly found. The obtained efficiency enables the single fiber theory to calculate the average penetration of diffusive particles through mechanical filter media under the cyclic flow conditions.