This study aimed to evaluate the effects of Sel-Plex on rumen fermentation and microbial protein synthesis in the rumen of steers. Eight ruminally cannulated Simmental steers were used in a replicated 4 × 4 Latin square experiment. Treatments were: control (without Sel-Plex), Se-low, Se-medium and Se-high with 7.5, 15 and 22.5 mg Sel-Plex per steer per day, respectively. Ruminal pH, total VFA, acetate and butyrate concentration were not affected by the treatment. The ratio of acetate to propionate was linearly reduced with increasing Sel-Plex dose, due to increased propionate production. Ruminal microbial protein synthesis was linearly and quadratically increased with increasing Sel-Plex dose. The results indicate that Sel-Plex potentially improves rumen fermentation. The optimum dose was 15 mg/d under the present experimental conditions.