ESSENTIAL oils from Cinnamomum verum (Cinnamon), Lavandula angustifoli (Lavender), Lepidium virginicum (Peppergrass oil), Zingiber officinale (Ginger) and Cinnamomum camphora (Camphor) have been investigated for their antifungal and antibacterial activities against 6 fungal isolates of Aspergillus flavus and 25 bacterial isolates of Klebsiella pneumoniae. The sensitivity was determined using agar well diffusion method and the inhibition zones were compared with the standard drug itraconazole for fungi and Amikacine for tested bacteria. The tested essential oil showed a wide spectrum of inhibition against the tested isolates. Treating the Klebsiella pneumoniae, with cinnamon essential oil led to an external changes, irregular cell shape and disintegration of bacterial cell wall under transmission electron microscope. GC-MS technique was used for cinnamon essential oil; the composition of cinnamon oil was dominated by cinnamaldehyde. The in vivo efficacy of cinnamon essential oil in treatment of Klebsiella pneumoniae in mice lungs was proven.