Abstract: Plant tannin is a kind of phenolic secondary compound, which is widely found in plants. Due to its active chemical properties and special molecular structure, tannins can produce anti-nutrition effects on ruminants, therefore, tannins have long been labeled as anti-nutritional substances. However, recent studies have shown that adding a certain proportion of tannin to ruminant diet can protect rumen protein, increase milk yield, prevent ruminal bloat, inhibit parasitic diseases and so on, which suggest that the effect of tannins on ruminants depend on the content of tannins in the diet. This paper summarizes the adverse and beneficial effects of tannin, and the application of tannin as well, on ruminants to provide theoretical basis and technical reference for rational utilization of plant tannin.