Abstract This paper provides some information about mathematical modelling that was prepared for natural attenuation of petroleum hydrocarbons contaminated soil. Concentration of initial contamination with hydrocarbons and time that is needed to degrade it was compared with the results that were received from experimental research of the bioremediation of hydrocarbons in soil by the use of silica nanocomposite. The aim of the mathematical modelling was to check if microorganism that was used in previous research was effective enough when their effectiveness is compared with natural attention effectiveness. The results of natural attenuation shows that in some cases in-situ remediation might be more effective than ex-situ bioremediation, however the effectiveness of natural attenuation depends on many factors and due that the results might vary in different areas.