This paper is focused on analyzing the engineering properties and geotechnical changes of the soil contaminated by petroleum hydrocarbons, compared to non-contaminated soil. Also, identifying the physical and chemical behaviors and the interaction mechanisms between soil and contaminants at the time of contamination is far-reaching. This study aims to investigate the effects of petroleum contaminants on Atterberg limits, cation-exchange capacity, electrical conductivity and acidity, maximum dry density, and optimum moisture content of clay soil compared to non-contaminated samples with similar geological conditions in Gachsaran petroleum refinery region. The significant effects of contaminants on clay soil include an increase in cation-exchange capacity and electrical conductivity, but a decrease in acidity compared to non-contaminated samples. The results of the Atterberg limit as a physical index of the soil showed an increase in the petroleum hydrocarbon concentration, which enhanced the plastic and liquid limits, but decreased the plasticity index. The results of compaction tests using water and oil fluids indicated that the concentration of petroleum hydrocarbons in sandy soil increased maximum dry density and decreased optimum moisture content. Moreover, in a soil sample with stable pollution levels, maximum dry density and optimum moisture content were higher and lower in the compaction test using water fluid compared to those in compaction tests using oil fluid, respectively. Finally, increasing the soil pollution level reduced the difference between maximum dry density and the optimum moisture content in the compaction test using water fluid compared to the same test using oil fluid.