Studding the diversity of soil indigenous microorganisms, and monitoring effect of contaminants on microbial population, is very critical for understanding microbial activity during bioremediation and selecting successful remediation strategy. To simulate the natural environment, four microcosms were prepared by artificially contaminating clean soil with defined amounts of petroleum hydrocarbons including alkanes mixture (C₁₃–C₂₀), polyaromatic hydrocarbons (PAHs) mixture (anthracene, phenanthrene, fluoranthene, pyrene and benzo (α) pyrene) and both alkanes and PAHs. Contaminants degradation and heterotrophic bacterial count were measured during a 6-month study. Copy number of alkB and C23DO genes was studied using real-time PCR, and bacterial diversity was monitored by 16S rRNA gene PCR and denaturing gradient gel electrophoresis (DGGE). Results indicated that all types of contaminants (except the five ring benzo (α) pyrene) were totally degraded after 6 months and the increase in hydrocarbon degradation rate coincided with the enhancement of total heterotrophic bacterial count in each microcosm. Real-time PCR results showed a significant increase in the copy number of both alkB and C23DO genes in alkane- and PAHs-contaminated microcosm comparing with the control microcosm, indicating selection for special hydrocarbon degraders in hydrocarbon-amended microcosms. The results of DGGE revealed that the type of contaminant in the same soil has a remarkable influence on soil bacterial community structure. Sequencing of DGGE bands suggested that most of the dominant members of the microbial community of contaminated soil are unculturable bacteria from Proteobacteria and the genus Bacillus.