Pyrene is a common petroleum contaminant. This compound is recalcitrant to biological degradation and persists long in contaminated environments. A microcosm experiment was conducted to investigate the degradation rate of pyrene in three different of soil : rhizosphere soil ; non-rhizosphere soil ; and sterilized soil. The degradation rate followed the order of rhizosphere soil)non-rhizosphere soil)sterilized soil. And the rate did not change significantly when organic acids commonly found in the rhizosphere were added to each soil but it seemed to be well related to the increase of the number of microorganisms. Overall, it appears that pyrene is degraded faster in the rhizosphere soil which has the higher microorganism density.