The pre-mRNA of eukaryotic cells must undergo extensive and complex processes to produce mature mRNAs, including 5" end capping, splicing and 3" end processing. Among them, 3" end processing includes cleavage and polyadenylation, which is controlled by the cis-elements of pre-mRNA and a number of protein factors. Mammalian 3" end processing machinery consists of cleavage and polyadenylation factors, cleavage and stimulating factors, cleavage factor Ⅰ and cleavage factor Ⅱ. Other protein factors include poly(A) polymerase, poly(A) binding protein, symplekin and so on. Mammalian genes usually contain multiple polyadenylation sites, and alternative polyadenylation can not only produce mRNA variants with different length of 3"UTR, but also change the CDS region of genes. As a key mechanism for the regulation of gene expression in eukaryotes, alternative polyadenylation plays an important role in cell growth, proliferation and differentiation. This paper reviews the formation of the 3" end of mammalian pre-mRNA, the composition and function of the 3" end processing machinery, the mechanism of alternative polyadenylation in various human diseases, and hope to bring some new insights to readers.