Leymus mollis (2n = 4x = 28, NsNsXmXm) is an important tetraploid species in Leymus (Poaceae: Triticeae) and a useful genetic resource for wheat breeding because of the stress tolerance and disease resistance of this species. The development of Triticum aestivum (common wheat) - L. mollis derivatives with desirable genes will provide valuable bridge materials for wheat improvement, especially regarding powdery mildew resistance genes, which are rarely documented in L. mollis. In the present study, three derivatives of common wheat cultivar 7182 and L. mollis, namely M47, M51, and M42, were subjected to chromosomal characterization via cytogenetic identification, the analysis of molecular markers, and genomic in situ hybridization. These derivatives were all morphologically and cytogenetically stable. M47 was highly resistant to powdery mildew and nearly immune to stripe rust at the adult stage, and the chromosome constitution of this derivative can be expressed as 2n = 56 = 42T.a + 14L.m (where T.a = T. aestivum chromosomes; L.m = L. mollis chromosomes). Compared to M47, M42 was also resistant to stripe rust but was susceptible to powdery mildew; the chromosome constitution of M42 was 2n = 54 = 42T.a + 12L.m, in which a pair of homoeologous group 7 L.m chromosomes was eliminated. Finally, M51 was susceptible to powdery mildew and stripe rust and had a chromosome constitution of 2n = 48 = 42T.a + 6L.m, in which four pairs of L.m chromosomes from homoeologous groups 2, 4, 5, and 7 were eliminated. The differing disease resistances of the three derivatives are discussed in this report in the context of their chromosomal variations; this information can thus contribute to breeding disease resistant wheat with the potential for applying these derivatives as useful bridge materials.