In this paper,we investigated the allelopathic effects of cinnamic acid and caffeic acid and their phytotoxic mechanism on lettuce seedlings,in order to reveal the mechanism of allelopathy by phenolic acids.The results showed that:(1)root length,shoot length and fresh weight of lettuce seedlings were all obviously inhibited by cinnamic acid at different concentrations,and the inhibition rate of root length,shoot length and fresh weight at concentration of 1 000 μmol/L was 89%,74% and 49%,respectively.Root length and shoot length of lettuce seedlings both were promoted at low concentration (0.1,1.0 and 10 μmol/L) and inhibited at high concentration (100 and 1 000 μmol/L) by caffeic acid.Besides,the growth inhibition of cinnamic acid on lettuce seedlings was obviously stronger than that of caffeic acid at high concentration.(2)Cinnamic acid and caffeic acid both had no significant effect on cell viability in lettuce root tips at low concentration (0.1,1.0 and 10 μmol/L).High concentration (100 and 1 000 μmol/L) of cinnamic acid and caffeic acid both significantly increased the number of dead cells.(3)Low concentration of cinnamic acid (0.1 and 1.0 μmol/L) and caffeic acid (0.1,1.0 and 10 μmol/L) both had no significant effect on the accumulation of reactive oxygen species (ROS).When the concentration of cinnamic acid was over 10 μmol/L and the concentration of caffeic acid was over 100 μmol/L,the accumulation of ROS increased rapidly with the increasing of concentration.ROS was induced and accumulated in lettuce seedlings under the stress of cinnamic acid and caffeic acid,which leaded to the reduction of cell viability and finally affected the growth and development of lettuce seedlings.Furthermore,the inhibition effect of cinnamic acid was significantly stronger than that of caffeic acid.