Abstract: The effects of magnesium propionate on lactation performance and metabolic parameters in early lactation Holstein dairy cows were evaluated using thirty-six multiparous Holstein dairy cows. They were blocked by lactation number, corrected milk yield in last lactation, and expected calving date, then designated into four treatments (0, 50, 100, and 150 g/d) of magnesium propionate supplement in a randomized block experiment. Average feed intake, milk fat percentage, milk protein percentage, milk lactose percentage and milk dry matter percentage were not affected by magnesium propionate supplementation (P>0.05). Milk yield, feed efficiency, body condition and metabolic parameters of cows supplemented with magnesium propionate at 100 and 150 g/d were significantly (P<0.05) better than the controls as were energy balance and concentrations of glucose and insulin in the cow’s plasma. With magnesium propionate supplements of 100 and 150 g/d, the concentrations of non-esterified fatty acids and β-hydroxybutyrate in plasma of cows were lower than those of the controls (P<0.05). The concentrations of urine ketones were significantly (P<0.05) lower than in cows with a magnesium propionate supplement of 50 g/d and of the controls. The optimum dose of magnesium propionate supplementation was 100 g/d.