Abstract: Four, lactating dairy cows were randomly assigned according to a 2x2 Factorial arrangement in a 4x4 Latin square design to study supplementation of malate level at 500 and 1,000 g and cassava hay in high-quality feed block. The treatments were as follows: T1 = supplementation of high-quality feed block without cassava hay + malate at 500 g; T2 = supplementation of high-quality feed block without cassava hay + malate at 1,000 g; T3 = supplementation of high-quality feed block with cassava hay + malate at 500 g; T4 = supplementation of high-quality feed block with cassava hay + malate at 1,000 g, respectively. The cows were offered the treatment concentrate at a ratio to milk yield at 1:2 and urea-treated rice straw was fed ad libitum. The results have revealed that rumen fermentation and blood metabolites were similar for all treatments. The populations of protozoa and fungal zoospores were significantly different as affected by malate level and cassava hay supplementation. In conclusion, the combined use of cassava hay and malate at 1,000 g in high-quality feed block with concentrates containing high levels of cassava chip at 65% DM could highest improved rumen ecology and nutrients digestibility in lactating dairy cows.