In Vietnam, the protection of water quality in general and in river basins in particular has become an issue of utmost concern in recent years as the consequences of the rapid socio-economic development. In this study, a multimetric index was developed and applied to assess the biological water quality of rivers with case study at Cau river basin as a complementary tool to the traditional physical-chemical monitoring. Five indices, including the Biological Monitoring Working Party (BMWP) Viet, total number of taxa, Margalef index, number of Ephemeroptera, Plecoptera and Trichoptera (EPT) , and Percent of insects, were selected to calculate a multimetric macroinvertebrate index (MMI) by arithmetic mean. The MMI ranged from class one (high biological status) to class five (bad biological status). The study demonstrated that the multimetric approach is suitable for application in the Vietnamese national monitoring and assessment program.