The genome sizes of 13 species of Taxodiaceae, 19 species of Cupressaceae s.s. and Sciadopitys verticillata were determined by flow cytometry of isolated nuclei stained with propidium iodide, using Hordeum vulgare nuclei as an internal standard. In Taxodiaceae, the genomes of Cunninghamia lanceolata (28.34 pg/2C) and Taiwania species (25.78, 26.80 pg/2C) were larger than those of other genera/species, which ranged from 19.85 to 22.87 pg/2C. In Cupressaceae s.s., genome size ranged from 20.03 to 27.93 pg/2C among 16 species. The Calocedrus species and Thujopsis had a larger genome than most other species. Sciadopitys verticillata had a large genome of 41.60 pg/2C. After comparing the diversity in genome size with previously reported cladograms constructed using nucleotide sequence data, the tendency of changes in genome size with phylogenetic differentiation is discussed.