© 2020 SAABIn this study we investigated antimicrobial activity against Klebsiella pneumoniae and the phagocytic functions of human leukocyte cells as revealed in an in vitro experimental model combining cefepime and gentamicin with Pelargonium endlicherianum Fenzl. essential oil treatments. The bactericidal effects of this essential oil and antibiotic combinations were dynamically detected by time-kill assay. To examine the function of this essential oil and antibiotics in permeating outer membrane barriers when used singly or in combination, a UV spectrophotometer was used, and morphologic images were captured by scanning electron microscopy. The antibacterial activity of the essential oil and antibiotics was assessed using broth microdilution and agar well diffusion. The combined effects of the essential oils of P. endlicherianum and gentamicin and cefepime were evaluated by means of the checkerboard method against K. pneumoniae. In the assays, fractional inhibitory concentration (FIC) values were calculated to characterize the interactions between the combinations. In the combinations of essential oil and antibiotics, the sensitivity of the bacteria to antibiotics increased and the antibiotics had a synergistic effect, and the antibacterial effect on the microorganisms increased. The cefepime + essential oil pair tested showed a synergistic effect (FIC ≤ 0.5), but the gentamicin + essential oil pair did not (FIC > 0.5–4.0). Thus, the cefepime + essential oil pair has been found to exhibit a synergistic effect against K. pneumoniae compared to the gentamicin + essential oil pair. According to the results obtained here, the combined use of essential oils with antibiotics can be applied as a treatment strategy to reduce the use of antibiotics, reduce side effects, and possibly reverse antibiotic resistance to these microorganisms in light of the increase in multiple antibiotic resistance.