Abstract: An experiment was conducted to determine the effect of supplemental Zn (as Zn sulfate) on rumen fermentation in steers. Four ruminally cannulated steers were fed, in a 4×4 Latin square design, 2.8 kg/d concentrate and 4.2 kg/d rice straw as a basic dietary. Treatments consisted of the following : control (no supplemental Zn) and 25 or 50 or 100 mg Zn/kg DM (on DM basis of dietary) from Zn sulfate. Rumen samples were collected at days 22, 23 and 24 of experiment period. There were significant effects of supplementation of Zn on concentrate of acetate, propionate, and total VFA (p< 0.05). The concentrate of ruminal NH3-N and the VFA molar proportions were not affected by the addition of Zn to diets (p>0.05). However, for comparing control (no supplementation), the concentrate of acetate and total VFA were higher with supplementation of 50 or 100 mg/kgDM Zn (p< 0.05). These results indicated that Zn-supplemented at 50 or 100 mg/kgDM improved rumen fermentation in steers.