Inteins are internal segment peptides which can self-splicing at the protein level. Although inteins are found in all three domains of life, they are not evenly distributed among species and proteins. So the evolution of intein have been capturing much attention. 69 classical inteins were found through systematically searching nucleic acids database. The comparison of homologous protein and phyogenetic tree of inteins suggest that the evolution of inteins should combine two causes: lateral transmission and inheritance.