To examine the effects of different sources of Rumen-Degradable Carbohydrates on rumen fermentation, nitrogen metabolism and performance of Holstein dairy cows, four lactating Holstein cows fitted with rumen cannulae in a 4×4 Latin Square with four diets and four 28 day periods each, were employed. Experimental diets contained different levels of pure sucrose and corn starch in which sucrose was replaced at 0, 25, 50 and 75 g /kg DM for corn starch at 75, 50, 25 and 0 g/kg DM in a total mixed ration (TMR) respectively. Replacing corn starch with sucrose did not affect ruminal pH. The addition of sucrose to TMR compared with starch, reduced (P?0.05) ruminal NH3 - N concentration but did not significantly affect peptide – N concentration. The addition of sucrose to diets did not significantly affect either total volatile fatty acids or the acetate to any propionate ratio. No differences in molar proportion of most of the individual volatile fatty acids were found among diets, except for the molar proportion of butyrate that was increased (P?0.05) with the inclusion of sucrose. In this experiment, diets with high levels of sucrose as compared with diets containing corn starch tended (P?0.051) to decrease total branched chain volatile fatty acids. Dry matter intake, milk yield, body weight changes and digestibility of DM, OM, CP and NDF were not affected by inclusion of sucrose in the diets but increased milk fat and total solid percentages (P?0.05). Increasing levels of sucrose in the diets tended (P?0.063) to increase milk protein percentage (3.28 vs. 3.05) and reduced (P?0.05) milk urea nitrogen concentration which shows improving nitrogen metabolism in the rumen and as a result more efficient utilization of the rapidly available nitrogen components in the diets.