Abstract: The knowledge of impacts of polycyclic aromatic hydrocarbons (PAHs) on enzyme activities in plant will be of great significance to risk assessment of PAH contamination. However, little information is hitherto available in literature on in vitro experiment to study PAH-influenced enzyme activity in plant. Impacts of PAHs on phenol oxidase (PPO) activity in shoots of clover (Trifolium repens) were investigated in vitro. Acenaphthene and pyrene were used to represent PAHs. PPO activity in clover shoots was enhanced steadily with rising acenaphthene concentration from 0 to 39.68 mg·L^-1, indicating an inductive effect of acenaphthene on PPO activity. In contrast, PPO activity increased first and decreased thereafter with increasing pyrenecon centration from 0 to 0.16 mg·L^-1, suggesting an inductive effect in the early phase and an inhibition effect in the late phase. It was observed that PPO in clover shoots was more sensitive to acenaphthene stress than to pyrene stress, which is consistent with the trend of acenaphthene being higher than pyrene in toxicity. All the results of this work indicate that in vitro experiment may be a fast, simple and effective means to evaluate impacts of PAHs on PPO activity in plants.