A number of Chinese, Japanese and European silkworm strains with coarse size filament were used for breeding the coarse size variety. Through cross breeding combined with pedigree selection within thirteen generations, a pair of coarse size silkworm variety, named ${CU_1}{ imes}{CU_2}$, was obtained after five years. Laboratory trials showed that the filament size of its 4F_1$ hybrid was over 4.3 D, and the other economic characters were also good.