Genome sizesof a Begonia collection comprising34 species (including fourvarieties) were screenedby flow cytometry, usingOryza sativa L. ssp. Japonica as anexternal reference, the differences in genome size between different speices and sections were compared, correlation between genome size and chromosome number was analysed. The results showed that 1C values varied between 0.292pg and 2.554pg, a 9-fold difference was found among genotypes with genome size, the meangenome size of 34 Begonia species was 0.863 pg, the genome of Begonia peltatifolia was smallest, the genome of B. fomosana origined from Taiwan was largest. The mean genome size of 30 Begonia species in China origin(1C=0.925pg) was lager than 4 Begonia species’ in South American origin(1C=0.398 pg), genome sizes of 3 Begonia species inTaiwan origin were all larger than Chinese origin Begonia species’. There were significant differences in the genome size between different sections of Chinese Begonias, and in the same section, the genome sizes of Begonias were not same. The mean genome size of Section Sphenanthera was largest, 1.285 pg, varied about 3.2 times; while Section Begonia, Section Platycentrum, a bit larger, which were 0.895 pg and 0.888 pg respectively, varied almost 6.4, 6.8 times; the meangenome size of Section Coelocentrum was smallest, 0.721 pg, varied about 1.2 times. Genome sizes were not positively correlated with chromosome number. The resu