Oil sludge is a viscous material consisting of resin, asphaltene, sand, and water, which is usually formed at the bottom of oil reservoir tanks. Asphaltene and resin contents of oil sludge make it more resistant to biodegradation. Disposal of oil sludge is the main problem of oil industry; discharge of oil sludge into the soil causes damage to the environment. Bioremediation is an efficient, cheap, and environmentally friendly method for oil-sludge treatment. The aim of this study was to investigate the biodegradation of oil-sludge fractions in contaminated soil for a period of 12 months. The oil sludge was mixed with soil with the final concentration of 5 % (w/w), and the nutrients such as phosphate and nitrate salts were added to soil. Finally, reduction of aliphatic, aromatic, resin, and asphaltene was tested. The results showed that the reduction of total petroleum hydrocarbon was 31 ± 5 % during 12 months of the treatment. About 60 ± 8 % of total aliphatic fractions, mainly C14–C22, decreased. Analysis for the detection of two, three, and four rings of polycyclic aromatic hydrocarbons demonstrated about 42 ± 3 % reduction of total aromatic fractions, whereas the resin (6 ± 0.8 %) and asphaltene (4 ± 0.5 %) fractions were slightly biodegraded. In conclusion, biotreatment of oil sludge during 12 months could well reduce aliphatic and aromatic fractions, but more time is needed for resin and asphaltene reduction.