Tropical grasses mainly utilized in southwestern Japan exhibited that the dry matter yield is increased as the growth stage. However, the nutritive value, such as crude protein (CP) and non-fibrous carbohydrate (NFC) decreased strikingly. Therefore, breeding for the new grass or crops with high potential as protein source is an urgent problem. The aim of the study were to investigate the dry matter yield of two soybean cultivars ('Miyakonojo' and 'Williams82') with Rhodes grass as living mulch plant after 1st cutting and to investigate the nutritive value of the silage. The growth of the Rhodes grass hampered the growth of soybean at the initial and the subsequent growth stages. Hence, the dry matter yield of soybean ranged from 9.8 kg/10a to 26.6 kg/10a, correspond to about 3 to 10% of total dry matter yield. The nutritive value of EE, CA and CP in Rhodes grass with soybean silage higher than those of Rhodes grass silage. These results suggest that the addition of soybean biomass could be useful for increasing the nutrition value of silage. Moreover, it is necessary to choose an appropriate sowing date to increase dry matter yield of soybean biomass in southwestern of Japan.