Mulberries (Morus spp.) include species that are generally diploid but may also occasionally exist in the triploid state (“diploid” species), and others that naturally have different levels of ploidy (“polyploid” species). In the present study, we measured the nuclear DNA amounts in 271 cultivars or strains of 15 mulberry species (nine diploid and six polyploid species) using flow cytometry. A wide range of interspecific 2C DNA values was observed, with the largest being 10.8 times higher than the smallest. Intraspecific differences in 1Cx values, a measure of the monoploid genome size, were within 11% for all species examined. Interspecific variation in 1Cx values was within 28% for all mulberry species in this study. The variation of 1Cx values in polyploid species was larger than that in diploid species. Among the diploid species, the 1Cx values of species native to the Japanese islands were higher than those that originated from East Asia on the Asian continent. The 2C values of other species of Moraceae and Cannabaceae were also estimated, and a wide variation was found.