Abstract: An experiment was conducted to determine the effects of dietary concentrations of copper on nitrogen metabolism, VFA production and fermentation of mixed ruminal microbes grown in continuous culture fermentors. Six fermentors were fed 40 g of diet. The diet consisted of concentrate and rice straw with Cu supplemented as CuSO4. Dietary treatments were 1) control (no supplemental), 6mg, 12mg, 18mg, 24mg and 30mg of supplemental Cu/kg of DM. The ruminal pH was not greatly affected by Cu supplementation. Ruminal fluid ammonia concentration in continuous ruminal cultures were affected by Cu supplementation (p<0.05). However, concentrations of ammonia increased (p<0.05) in response to supplemental Cu. The fermentation of organ matter was increased by the supplemental Cu. The ruminal fluid VFA concentrations were affected by Cu supplementation (p<0.05). The concentrations of acetate, propionate and butyrate increased (p<0.05) in response to supplemental Cu. Molar proportions of acetate, propionate and butyrate were not affected by the addition of supplemental Cu to the basal diet. The results indicate that addition of supplemental Cu to the control diets improved growth of ruminal microbe, and increased ruminal fermentation of organ matter and concentrations of VFA in continuous cultures.