Benzo(a) pyrene (BaP) can't be biodegraded in the soil easily, the degradation of BaP is know as a cometabolic process. This paper dealed with the effects of some polycyclic aromatic hydrocarbons (PAHs) and two kinds of monoaromatics on the degradation of BaP. The test soil was pre-incubated with the aromatics for 3d and spiked with BaP stock solution, which eventually formed into slurry system in Erlenmeyer flasks and was incubated on rotary shaker. 42 d experiment demonstrated that lag period in degradation process of BaP disappeared due to the preexposure to phenanthrene, and degradation rate of BaP was enhanced. On the contrary, degradation pattern of BaP was not obviously improved by the preexposure to anthracene or pyrene, and was even inhibited by benz(a) anthracene. The lag period also disappeared after the preexposure to salicyclic acid or phthalic acid, and the degradation of BaP was accelerated by both monoaromatics, even though the impact of mass difference on the degradation of BaP was not detected.