The objective of this experiment was to study the effect of feeding pellet containing high sulfur (PELFUR) diet and fresh cassava root (FCR) to Thai native beef cattle on feed use efficiency, ruminal characteristics, and blood metabolites. Four male Thai native beef cattle (150 ± 15.0 kg of body weight (BW)) were allocated with a 2 × 2 factorial arrangement in a 4 × 4 Latin square design. Factor A was FCR supplementation at 15 and 20 g/kg of BW. Factor B was the sulfur level in the PELFUR ration at 15 and 30 g/kg of dry matter (DM). No interaction effect was found among FCR supplementation and PELFUR in terms of feed intake and nutrient intake (p > 0.05). Cyanide intake was significantly increased based on FCR supplementation (p < 0.05), whereas sulfur intake was increased by level addition of PELFUR levels (p < 0.05). There were interaction effects among FCR supplementation and PELFUR on digestibility coefficients of DM and organic matter (OM) (p < 0.05). FCR supplementation at 20 g/kg BW with PELFUR 30 g/kg demonstrated the highest digestibility of DM and OM. Moreover, interactions were observed between FCR and PELFUR for bacterial populations (p < 0.01). The populations of bacteria were highest in FCR supplementation at 20 g/kg BW with PELFUR 30 g/kg at various feeding times. An interaction effects from among feeding FCR with PELFUR was found on blood thiocyanate concentrations at various feeding times (p < 0.01). The highest mean values of blood thiocyanate were observed when feeding FCR at 20 g/kg BW with PELFUR at 30 g/kg. No interaction effect was found between FCR and PELFUR on total volatile fatty acids (VFA) and their profiles (p >0.05). However, the proportions of the total VFA at 0 and 4 h post-feeding were increased when FCR at 20 g/kg BW was supplemented (p < 0.01). FCR at 20 g/kg BW could enhance propionate (C3) at 4 h post-feeding when compared with FCR at 15 g/kg BW (p < 0.01). Moreover, supplementation of PELFUR at 30 g/kg increased the total VFA at 0 and 4 h post-feeding, whereas the concentration of C3 at 4 h post-feeding was enhanced (p < 0.05). However, no significant changes were found for any parameters among treatments and between the main effect of FCR and PELFUR supplementation (p > 0.05). In conclusion, feeding of two combinations (FCR 20 g/kg BW with PELFUR 30 g/kg) could promote the nutrient digestibility, the bacterial populations, and the rate of disappearance of cyanide without having any adverse effect on rumen fermentation.