The current study was conducted to evaluate the effect of monensin on the digestibilities of dry matter (DM), organic matter (OM) and crude protein (CP), ruminal fermentation and the degradation kinetics of a basal diet (control) containing soybean meal as a protein source. Three ruminally fistulated male sheep were used in a 3x3 Latin square design experiment to be fed on three rations; 1) control diet, 2) control diet plus monensin at 30 mg/day and 3) control diet plus monensin at 50 mg/day. The results showed that the addition of monensin to the diet had no significant (P>0.05) effect on DM, OM and CP digestibilities. When animals were fed monensin diets, the concentration of ammonia-N (NH₃-N) was significantly (P<0.05) decreased from 14.6 to 12.15 and 11.99 mg/dl, for control diet and control diet plus monensin at 30 mg/day and control diet plus 50 mg/day, respectively. The molar proportion of propionic acid was significantly (P<0.05) increased, while the molar proportion of acetic acid was significantly (P<0.05) decreased with the addition of monensin. Butyric acid was not significantly affected with the addition of monensin to the control diet. The effective degradability of DM and CP was significantly (P<0.05) decreased with the addition of monensin to the control diet. Microbial nitrogen leaving the rumen was significantly (P<0.05) increased with the addition of monensin. These results suggested that the addition of monensin to the diet including soybean meal as a protein source could reduce ruminal protein degradation and enhance the amount of total protein that reaches the post-ruminal tract.