This paper deals with the results of the karyological studies on seventeen moss species belonging to the family Thuidiaceae. In the karyological studies, metaphase and heteropycnotic chromosomes in mitotic cells of gametophyte were observed. In sixteen species of the family, excluding Tetracladium molkenboeri (n=22), the same chromosome number, n =11 was counted. The karyotypes of these sixteen species were so much alike with each other that they were represented by the same karyotype formula. The karyotype formulae of the seventeen species are as follows: Haplohymenium pseudo-triste H. sieboldii H. triste Herpetineuron toccoae Claopodium aciculum C. assurgens C. gracillimum C. nervosum C. pellucinerve Haplocladium microphyllum H. strictulum H. subulaceum Thuidium bipinnatulum T. cymbifolium T. glaucinum T. kanedae}K(n)=11=V(H)+V+J+7(5v+2j)±m(h) Tetracladium molkenboeri……K(n)=22=2V(H)+2V+2J+14(10v+4j)+2m(h).