Abstract: Four, one-year old of dairy heifers were randomly assigned according to a 2x2 Factorial arrangement in a 4x4 Latin square design to study supplementation of malate level at 500 vs 1,000 g with yeast at 1,000 vs 2,000 g in concentrate. The treatments were as follows: T1 = supplementation of malate at 500 g + yeast at 1,000 g; T2 = supplementation of malate at 500 g + yeast at 2,000 g; T3 = supplementation of malate at 1,000 g + yeast at 1,000 g; T4 = supplementation of malate at 1,000 g + yeast at 2,000 g in concentrate, respectively. The cows were offered the treatment concentrate at 1 %BW and ruzi grass was fed ad libitum. The results have revealed that rumen fermentation and blood metabolites were similar for all treatments. However, the concentration of volatile fatty acid was significantly different especially the concentration of propionic acid was slightly higher in heifer receiving T4 than T3, T2 and T1 (24.4, 22.9, 22.4 and 19.7%, respectively). The populations of protozoa and fungal zoospores were significantly different as affected by malate level and yeast. In conclusion, the combined use of concentrate containing high level of cassava chip at 70% DM with malate at 1,000 g and yeast at 2,000 g in concentrate with ruzi grass as a roughage could improved rumen ecology in dairy heifers.