Abstract: Allelochemicals are released into the farmland soils by plants, which will affect the growth of other plants. Phenolic acids are one kind of important allelochemicals. Due to their small molecular weight and stable structure, phenolic acids are hardly decomposed in nature. Thus they would accumulate in the soil, and affect the crop production during a long period of time. In fact, phenolic acids are the main compounds causing the problem of continuous cropping obstacle. In order to evaluate the allelopathic effect and study the mechanism of action of phenolic acids, the allelopathic effects of benzoic acid, salicylic acid and 4-hydroxybenzoic acid were assayed using lettuce as the receptor. In addition, the influence on the cell viability and reactive oxygen species (ROS) accumulation of lettuce roots by the phenolic acids were studied with FDA/PI double staining and DHE staining, respectively. The results showed that the phenolic acids promoted the root and shoot elongation at low concentrations, and exhibited inhibitory effects at high concentrations. Moreover, under high concentrations, the above three allelochemicals could induce the accumulation of ROS. High concentrations of ROS might cause membrane structural damage and then influenc the cell viability, which will ultimately result in the growth inhibition of lettuce seedlings.