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Petroleum contamination of soil is a serious problem throughout the oil producer countries. Vegetation may play an important role in the biodegradation of toxic organic chemicals in soil. For petroleum compounds, the presence of rhizosphere micro flora may accelerate biodegradation of the contaminants. In a greenhouse study, petroleum contaminated soil were treated using phytoremediation . The C/N ratios and microbial populations were assessed in the beginning and the end of trials. The results showed that Puccinellia distance could tolerate the harsh condition of the soils. As MPN increases, C/N ratios decrease among trials. N, as a nutrient, had effects on both microbial populations and decreasing of organic carbon. Among six C/N ratios, organic carbon content of soil was lower at the end of the study in the vegetated pots compared with the non-vegetated ones. For the most part, the presence of plants enhanced the dissipation of the contamination. Our findings show that in level of 12.9 g crude oil per kg of dry soil we have critical point and for successful pyhtoremediation operation pollution must be below this amount. Microbial activity in loam texture had greater numbers and seedling pots had better efficiency in comparison with planting seed pots.