Synthetic compounds mimicking the structure of natural antimicrobial peptides (AMPs) have a great promise as potential anti-infectious agents due to their stability towards enzymatic degradation, high antibiotic efficiency, and broad adjustability of physicochemical properties. Recently we have demonstrated that antimicrobial activity of AMP synthetic analogs depends on their conformational rigidity. Cyclization is one of the strategies to restrain the flexibility of antimicrobial agents. Herein we present results of a study aimed to establish how the cyclization affects the ability of cyclic N- substituted glycine oligomers (peptoids) to disrupt selectively bacterial, but not mammalian cell, membrane mimics. Lipid monolayers at the air/liquid interface composed of LPS or DPPG were used to model the outer leaflets of Gram-negative and Gram-positive bacterial membranes, respectively, while the DPPC/Cholesterol 6/4 mixed film was used to mimic the mammalian plasma membrane. Interactions of cyclic and linear peptoids with model lipid membranes were investigated using constant-pressure insertion assays, epifluorescence microscopy (EFM), and synchrotron X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD). Insertion assays show that both cyclic and non-cyclic peptoids readily incorporate into the bacterial, but not mammalian, membrane mimics. Moreover their insertion into the bacterial membrane mimics was accompanied by rapid deterioration of the structural order in the lipid acyl chains. Electron density profiles across the film, derived from XR data, demonstrate that both peptoids penetrate into the hydrophobic core of DPPG more efficiently than that of LPS, which might be due to a difference in packing of the hydrophobic core. Nevertheless, our data indicate that, despite these similarities, the mechanisms of action of cyclic and linear peptoids on bacterial membranes are different.