found that the inclusion of U-cas at 180 g/kg DM in the feed block could improve in vitro rumen fermentation, microbial mass and digestibility. However, there is still a limitation of data on the effect of urea replacement by U-cas in the feed block in in vivo work. Therefore, the present study was undertaken to investigate the effect of different levels of U-cas inclusion in the feed blocks on feed intake, digestibility of nutrients and rumen fermentation in Thai native beef cattle fed on rice straw based diet.2.