Abstract: Four, one-year old of dairy heifers, weighing at 200±10 kg were selected. Cows were randomly assigned according to a 4 x 4 Latin square design to study supplementation levels of Yeast Fermented Cassava Chip (YFCC) replaced concentrate on rumen ecology, cost production and average daily gain. The dietary treatments were as follows: T1 = supplementation of concentrate: YFCC ratio at 100:0; T2 = supplementation of concentrate:YFCC ratio at 75:25; T3 = supplementation of concentrate:YFCC ratio at 50:50; T4 = supplementation of concentrate:YFCC ratio at 25:75, respectively. The animals were offered the treatment concentrate at 1.5 %BW and rice straw was fed ad libitum. The results have revealed that feed intake and average daily gain cost productions were significantly different among treatments especially affected the rice straw intake and average daily gain were higher in dairy heifers receiving T3 than T4, T2 and T1. In contrast, the cost productions was lower in dairy heifers receiving T3 than T4, T2 and T1. However, the rumen fermentation and blood metabolites were similar for all treatments. The populations of protozoa and fungal zoospores were significantly different as affected by levels of yeast fermented cassava chip supplementation. These results suggest that supplementation of yeast fermented cassava chip could highest replace at 75% of concentrate in dairy heifers.