The chromosome number and karyotypes of seven species of Alnus in European and American areas were studied by shedding cell wall and dialysis methods.The results showed that the karyotypes among them were not distinct and most chromosomes were metacentric or submetacentric.The chromosome number of A.cordata (hexaploid) was 2n=6x=42,the karyotype formula was 2n=6x=42=36m+6sm;2n=8x=56 and 2n=8x=56=46m+10sm(SAT) for A.viridis (octoploid);The chromosome number in somatocyte of A.tenuifolia,A.incana,A.glutinosa,A.sinuata and A.rubra all were 2n=4x=28,were tetraploid.The karyotype formula were 2n=4x=28=16m(SAT)+12sm,2n=4x=28=22m+6sm,2n=4x=28=24m+4sm,2n=4x=28=24+4sm,2n=4x=28=26m+2sm.A.rubra belonging to “1B” of Stebbins,the rest six species belong to “2B” of Stebbins.