Four Thai native beef cattle were randomly assigned according to a 4×4 Latin square design to evaluate the effect of urea calcium sulphate mixture (U-cas) levels in feed block on feed intake, apparent digestibility of nutrients and rumen fermentation fed on rice straw based diet. The four dietary treatments were different levels of U-cas supplementation in feed block at 0, 120, 150 and 180g/kg dry matter (DM), respectively. The present results revealed that rice straw intake increased with the increasing levels of U-cas inclusion in the feed block. The highest total intakes of DM and energy (ME, MJ/d) were in cattle receiving feed block containing U-cas at 180g/kg DM, and followed by 150, 120 and 0g/kg DM, respectively. Apparent digestibility of nutrients enhanced the increasing levels of U-cas supplementation in feed blocks, except acid detergent fiber. Rumen pH and temperature were not affected by U-cas levels inclusion in feed block. At 4h post feeding, the concentration of ruminal ammonia nitrogen decreased with increasing levels of U-cas in feed blocks (P<0.05). Inclusion of U-cas at 180g/kg DM in the feed block increased the ruminal propionic acid concentration at 4h post feeding while ratio of acetic: propionic acid and acetic plus butyric: propionic acid were lower than those in other groups (P<0.05). Inclusion of U-cas at 180g/kg DM as a slow release urea source in the feed block resulted in improving feeding value of the diets as rice straw base in Thai native beef cattle.