In spite of common properties duck orthoreoviruses (DRVs) are antigenically different from other avian orthoreoviruses (ARVs). We analyzed the S-class genome segments of the DRV S12 and compared them with S-class genome segments of other orthoreoviruses compared to those of ARV S1133. The full-length S-class sigmaA, sigmaB, sigmaNS, and sigmaC genes were determined and compared with other ARVs to study the degree of genetic divergence and evolution of DRVs. The alignment of the DRV S12 sigmaA, sigmaB, sigmaNS, and sigmaC genes with DRV 89026 showed 90.0%, 93.6%, 88.0%, and 93.1% nucleotide identity and 97.1%, 94.3%, 95.8%, and 93.7% amino acid identity, respectively. The alignment of the DRV S12 sigmaA, sigmaB, sigmaNS, and sigmaC genes with other ARVs revealed 76.0-77.1%, 52.5-55.1%, 78.4-79.6%, and 2.7-9.9% nucleotide identity and 89.5-91.2%, 61.4-62.0%, 91.6-92.7% and 22.6-26.7% amino acid identity, respectively. Phylogenetic analyses demonstrated that DRVs were quite different from other ARVs and provided the evidence for the diversity among avian orthoreoviruses.