Brassica oleracea contains various morphotypes within the species, but genomic signatures differentiating each morphotype have been poorly understood of. Here, we utilized whole genome sequence data of 44 B. oleracea collections including those of seven different morphotypes such as cabbage, broccoli, cauliflower, kailan, kale, brussels sprout, and kohlrabi to elucidate the genomic signature of B. oleracea morphotypes. Molecular structure analysis divided the 44 B. oleracea lines into two groups: group I represents broccoli, cauliflower, kailan; group II represents other B. oleracea subspecies. Kohlrabi has admixed genomic structure through genetic admixture analysis. Based on the population stratification result, we have investigated genetic signatures that offer the possible evolutionary processes for the kohlrabi morphotype. Several statistical analyses were implemented to identify selective regions and explore 45 candidate loci that may contribute to stem swollen in kohlrabi. Above all, we identified two kohlrabi-unique genes, LOC106333915 and LOC106308097, showing kohlrabi-unique non-synonymous mutations, which might be candidate genes for stem swollen in kohlrabi.