Abstract: To reveal the roles of phenylalanine ammonia-lyase (PAL) in low temperature tolerance in cucumber seedlings, a specific PAL inhibitor (AOPP) was sprayed to the seedlings, and then the stress tolerance was determined. The results suggested that both gene expression and enzymatic activity of PAL in cucumber leaves were induced under low temperature. The seedlings pretreated with AOPP showed lower PAL activity and less accumulation of phenolics and flavonoids. Low temperature caused damages in cucumber seedlings, and pretreatment of AOPP aggravated these damages. Compared to the control, the seedlings pretreated with AOPP showed significantly higher relative electrolyte leakage and MDA production, lower maximum photochemical efficiency of PSII (F_v/F_m) but higher photochemical quenching coefficient Y(NO)， and reduced expression of low temperatureresponsive genes (PR1-1a, COR47, P5CS and HSP70). In cucumber seedlings, low temperature stress induced the accumulation of H₂O₂, increased the contents of ascobate (AsA) but decreased the contents of dehydroascobate (DHA), and thus reduced the value of AsA:DHA. In the AOPPpretreated seedlings, the activities of antioxidant enzymes (CAT and APX) were significantly repressed, and excess H₂O₂ accumulated. The value of AsA:DHA was also lower than the control. Furthermore, coapplication of H₂O₂ scavenger alleviated the low temperatureinduced damages in the AOPPpretreated seedlings, while coapplication of a CAT inhibitor made the seedlings more sensitive to low temperature stress. These results indicated that under low temperature stress, the enhanced activities of PAL could increase the biosynthesis of phenylpropanoid compounds and activate the cellular antioxidant enzymes, which could scavenge the excess ROS and maintain the cellular redox status, and thereby reduce the photo and oxidative damages caused by low temperature stress.