Abstract: Object This study aimed to investigate functions of salicylic acid （SA） and its biosynthesis pathway on regulating nutlet abscission at the nut sizing stage of Carya illinoinensis （Wangenheim） K. Koch，so as to elucidate the internal causes of nutlet abscission，and provide a new perspectives for the high and stable yield of Carya illinoinensis （Wangenheim） K. Koch.Method The nuts of Pawnee variety of Carya illinoinensis （Wangenheim） K. Koch were taken as materials，the SA contents in abscised and normally developed nutlets were measured at 53 days after pollination to analyze effects of SA treatment on the nutlet abscising and development during the nut sizing stage of Carya illinoinensis （Wangenheim） K. Koch. Based on transcriptome data，the expressions of genes encoded by two key enzymes—phenylalanine ammonia-lyase （PAL） and isochorismate synthase （ICS）—were analyzed between the abscising and normally deve-loped nutlets. The gene structure， chromosome location and phylogenetic tree of PAL gene family members in Carya illinoinensis （Wangenheim） K. Koch were analyzed.Result The SA content in abscising nutlets was significantly higher than that in normally developed nutlets at 53 days after pollination （P<0.05， the same below）. The nut abscising rate of SA treatment group was highly significantly higher than that of control （water group） after SA treatment for 10 d （P<0.01， the same below）. The single nut mass in SA treatment group was significantly or highly significantly lower than that of the control group at after 7， 10 and 13 d of SA treatment. Under normal growth conditions， the single nut mass of abscising nutlets was significantly lower than that of the normally developed nutlets； all the developmentally retarded or arrested nuts will abscise within the following 7 d. The FPKM of genes CiPAL5 and CiPAL6 in abscising nutlets were significantly up-regulated comparing with normally developed nutlets. There were six PAL gene family members （CiPAL1-CiPAL6） in the Carya illinoinensis （Wangenheim） K. Koch genomes， and the three up-regulated CPAL4-6 genes with similar structures were closely arranged in tandem on Chromosome No. 4. Phylogenetic tree results showed that proteins CiPAL1-CiPAL3 shared a close relationship with PAL proteins of dicotyledons such as Arabidopsis thaliana， tobacco， grape， and poplar. Proteins encoded by CiPAL4-CiPAL6 tandem gene clusters located in Chromosome No. 4 of Carya illinoinensis （Wangenheim） K. Koch were cluster with proteins encoded by VviPAL3-VviPAL6 tandem gene clusters located in Chromosome No. 11 of grape.Conclusion SA treatment inhibits the nut development of Carya illinoinensis （Wangenheim） K. Koch and promotes nutlet abscising， suggesting that SA plays an important role in the progress of nutlet abscising during the nut sizing stage. PAL pathway is the main SA biosynthesis pathway in the process of nutlet abscising during the nut sizing stage and regulates the nutlet abscising of Carya illinoinensis （Wangenheim） K. Koch.