The relative abundance of dominant bacterial phyla during a biostimulation practice of petroleum contaminated soil has been evaluated through the Illumina sequencing method. Biostimulation practice was fulfilled by amending the petroleum contaminated soil with addition of different N/P ratios. Gram-negative bacteria were found to be more dominant than gram-positive bacteria. Proteobacteria, Firmicutes, Chloroflexi, Acidobacteria, Lentisphaerae, Planctomycetes, Thermotogae, Bacteroidetes, Synergistetes, and Verrucomicrobia were dominant phyla with various relative abundances in all of the studied microcosms. Among the dominant phyla, Proteobacteria was most significant. The relative abundance of Bacteroidetes and Verrucomicrobia significantly increased with an increase in petroleum hydrocarbons and nutrient concentrations, while the relative abundance of Firmicutes also increased with increasing nutrient amounts. Based on the results obtained from microcosm with the highest petroleum hydrocarbons removal rate, Proteobacteria, Firmicutes, and Bacteroidetes phyla, which composed about 50% of dominant phyla, proved to be the most influential on the biodegradation of petroleum contaminated soil. Among all dominant phyla, the relative abundance of Bacteroidetes was most affected by the increase in nutrient ratio, while the relative abundance of Acidobacteria showed least deviation among studied microcosms. (C) 2016 Elsevier B.V. All rights reserved.