A total of 61 chromosome counts from 11 taxa of Korean Spiraea was made. Our counts are the first report for eight taxa; S. blumei (2n=18, 36), S. chartacea (2n=36), S. chinensis (2n=36), S. fritschiana (2n=27, 36), S. microgyna (2n=18), S. prunifolia var. simpliciflora (2n=18), S. pseudocrenata (2n=36), and S. trichocarpa (2n=18). A new chromosome number of 2n=36 (tetraploid) is reported for S. pubescence. Populations of three species including S.blumei, S. pubescence and S. fritschiana, show different ploidy levels; diploid and tetraploid populations are found in the former two species and triploid and tetraploid ones, in latter species. Multiplication of chromosome numbers contributes to increase in size of pollenand stomata in the three species. Populations with different ploidy levels in S. blumei occupy different regiions; diploid populations in inland Korea and tetraploid ones in Ullung Island. Island tetraploid population of S.blumei might be originated from intra-island polyploidization through the introduction ofdiploid from inland Korea, considering the worldwide distribution of this species. Pollen fertilities of island populations of S. blumei are relatively low, and sometimes no pollen grain is produced in anther sacs; it suggests that tetraploid population of the island is gynodioecious which may serve reduction of inbreeding depression.