Fifty-four biological characters were used for quantitative classification of sorghum and sudangrass and their relationship. Discriminant analysis of all 240 individuals of 48 accessions suggested that 100% of originally grouped cases were correctly classified. Analysis of variance results showed that the differences among 48 accessions in characters were significant at the 0.01 level. Cluster analysis showed that the 48 accessions were obviously divided into two clusters by a Euclidean distance of 17.46 as the classification standards, i.e. the genus maize and genus sorghum. The 48 accessions were divided into four clusters by a Euclidean distance of 9.86, i.e. Sorghum-Sudangrass, Sorghum related species (S. propinquum and S. halepense), Zea mexicana and Z. mays. All accessions were divided into five clusters by a Euclidean distance of 8.91, namely group Ⅰ (Sudangrass-Chinese sorghum), group Ⅱ (Non-Chinese sorghum), group Ⅲ (Sorghum related species), group Ⅳ (Zea mexicana), and group Ⅴ (Z mays).