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Soil pollution by petroleum hydrocarbons is a widespread problem which has disturbing effects on the composition and diversity of microbial communities. The aim of this research is study the response of soil microbial communities to crude oil contamination. This research examines three microcosms: unpolluted, polluted and polluted with addition of (nitrogen and phosphorus as nutrients). Some factors were analyzed in each microcosms for 120 days include: enumeration of heterotrophic and degrading bacteria, enzyme assay and degradation of crude oil. Desert soil has an abundance in degrading bacteria (2 x 10(6) cfu/g), however, crude-oil degrading bacteria were lower than heterotrophic bacteria. The density of crude-oil degrading bacteria declines until the day 60th and after this time they increased. The highest dehydrogenase activities are found in the polluted microcosms amended with nutrients. Hydrocarbon degradation is lowest in the desert microcosm (40%). The results of this research confirmed a significant relationship between (Most Probable Number) of heterotrophic bacteria and degradation of crude oil. The results obtained in this research may be useful for developing strategies for bioremediation of pollution of different soil types.