Abstract: An experiment was conducted to investigate the effects of 2 levels of dietary Crude Protein (CP) in concentrates with similar proportions of Rumen Undegradable Protein (RUP) on rumen metabolism, milk yield and composition in mid lactating cows in Thailand. Eight 87.5% Holsteinx12.5% indigenous multiparous cows were used in a crossover design with two successive 25 days periods. Diets contained 30% paragrass and 70% concentrate on a Dry Matter (DM) basis. Concentrate feeds were formulated to provide low dietary CP (17.3%; LCP) or high dietary CP (19.04%; HCP). The proportion of Rumen Degradable Protein (RDP) and RUP was 61 and 39% in both diets. Diets were isocaloric in terms of net energy for lactation. Milk yield, milk lactose yield, Dry Matter Intake (DMI) and apparent digestibility of DM, CP and Neutral Detergent Fiber (NDF) were greater in cows fed HCP than in those fed LCP. Concentration of blood urea nitrogen was elevated in cows fed HCP diets. Rumen NH₃-N concentration and pH tended to increase in cows fed HCP diet. Rumen microorganism counts and volatile fatty acids levels in the rumen did not differ between treatments. The increasing CP content in mid-lactating cow was beneficial to increase DMI, apparent digestibility of DM, CP and NDF and therefore milk yield.