Abstract: The resistance of microorganisms issue to conventional antibiotics has necessitated the search for efficient and cost effective ways for treating infectious diseases. Plant essential oils are potential sources of novel antimicrobial compounds. In addition, the antimicrobial susceptibility testing provides an information aids in selecting and developing the appropriate antimicrobial agent. In the current study, the chemical compositions of the essential oils of Nigella sativa, Menthe piperita and Pelargonium graveolens (Geranium oil) collected in Egypt were characterized by GC-. In addition, the in vitro antimicrobial activity of these essential oils were tested, using agar diffusion method, against different eleven pathogenic microbial species including three Gram-positive, six Gram-negative and two fungi; a yeast like Candida albicans and a filamentous like Aspergillus niger. Inhibition zones showed that the essential oils of the two plants were active against all Gram-positive studied bacteria and fungi. The susceptibility of the strains changed with the dilution of essential oils in DMSO. The pure essential oils showed the most wide inhibition zones and they were very effective compared to standard drugs ciprofloxacin and/or nystatin. However, the activity against Gram-negative bacteria varied. Thus, this study indicates that the essential oils of Nigella sativa, Menthe piperita and Pelargonium graveolens have antimicrobial activity against different species of human pathogenic microorganisms and research should continue to examine the activity in experimental animals.