Volatile constituents of Opuntia ficus indica (L.) Mill (prickly pear or nopal cactus) cladodes, fruits peel and fruits pulp were prepared by hydrodistillation. The chemical composition was investigated by GC/MS analysis. Dodecane found as the major compound in the volatile constituents of fruits peel and cladodes representing (32.3%) and (20.05%) respectively followed by undecane (20.02%) in cladodes, (14.47%) in fruits peel while the fruits pulp volatile constituents contain undecane as a major compound (20.52%) followed by dodecane (16.86%). The antimicrobial activity was carried out for all samples using diffusion agar method against Aspergillus flavus, Candida albicans, Staphylococcus aureus, Bacillus subtillis, Escherichia coli, Salmonella typhimurium. Fruits peel volatile constituents showed antimicrobial activity against all tested microorganisms except Candida albicans while fruits pulp volatile constituents showed activity against Staphylococcus aureus and gram-negative bacteria and had no activity against fungi and Bacillus subtillis. Cladodes volatile constituents had no activity against all tested microorganisms.