To investigate the mechanism of russet skin formation in pear (Pyrus bretschneideri Rehd.),we used the exocarp of the wild type ‘Dangshansuli’ and its mutant type ‘Xiusu’ pear with russet fruits at different days after full bloom (DAFB) as material in this experiment.The content of polyamine was determined by High Performance Liquid Chromatography (HPLC).The key genes in polyamine biosynthesis were cloned by rapid amplification of cDNA ends (RACE),and their physicochemical properties were conducted by the protocol of Protparam website (http://web.expasy.org/protparam/) and the phylogenetic tree of putative proteins was constructed by MEGA5.0 software.The relative expressions of ADC,SPDS and SPMS genes at different stages were analyzed by using real-time quantitative reverse transcription-PCR (qRT-PCR).The results showed that:(1)The putrescine contents in the exocarp of ‘Dangshansuli’ was significantly higher than that in ‘Xiusu’ except for 50 and 150 DAFB.At 75 DAFB,the contents of spermidine and spermine in the exocarp of ‘Xiusu’ were higher than that in ‘Dangshansuli’.(2)The genes of arginine decarboxylase (ADC),spermidine synthase (SPDS) and spermine synthase (SPMS) were cloned,and were submitted to the database of the National Center for Biotechnology Information(NCBI) with the accession number KM923903,KM923905 and KM923906 ,respectively.The structure of putative proteins of ADC，SPDS and SPMS gene in pear was closer to that in apple.ADC and SPMS were all water-soluble by the prediction of their physicochemical properties.The SPDS was water-repellent by the prediction of their physicochemical properties.(3)The levels of relative expressions of ADC,SPDS and SPMS in the exocarp of ‘Xiusu’ were higher than that in ‘Dangshansuli’ at 50 DAFB.It was postulated that the metabolism of polyamine and up-regulation of relevant genes may be associated with the formation of russet fruit of ‘Xiusu’ pear.