Two experiments were conducted to study the effects of supplementation of rumen inert C18:1 (Megalac) and C18:0 (Energy Booster) in diets containing low fat (3.3% DM, experiment 1) and high fat (6.6% DM, experiment 2) on milk performance in early lactation cows. The high fat diets were formulated by replacement of 2% barely grain in low fat diet with soybean oil, and the contents of other ingredients were similar. Each experiment utilized 30 cows blocked by parity and predicted transmitting ability, and randomly fed diets either supplemented with rumen inert C18:1 or C18:0 from 3 to 14 wk after calving. Milk yield and DMI were recorded daily, and milk samples were collected weekly. Data were analyzed using a randomly blocked design with repeated measurements. When cows were fed diets containing 3.3% fat, the cows fed C18:0 supplement had more (P < 0.05) DMI by 2.4 kg/d, milk fat percentage by 0.3% unit, milk fat yield by 0.2 kg/d, milk protein yield by 0.07% unit, milk protein percentage by 0.1% unit, milk lactose percentage by 0.1%. The milk production efficiency (milk yield/DMI) of cows fed C18:1 supplement was greater (P < 0.05) than cows fed C18:0 supplement (1.9 vs 1.8). The milk yield, other milk components percentages and yields, net energy balance and BCS were similar. In milk, cows fed C18:1 supplement had greater (P < 0.05) concentrations of C18:1 cis-9, and cis-6, and cows fed C18:0 supplement had greater concentration of C18:0. In blood, the concentration of β-hydroxybutyrate was greater (P < 0.05) in cows fed C18:0 supplement. In experiment 2, the milk protein percentage of cows fed C18:0 supplement was greater (P < 0.05) than cows fed C18:1 supplement, while the DMI, milk yield, percentage and yield of other milk components were not affected. The milk production efficiency (milk yield/DMI) of cows fed C18:1 supplement was greater (P < 0.05) than cows fed C18:0 supplement (2.0 vs 1.9). The results of these studies suggest that the milk performance of early lactation cows supplemented with rumen inert FA varies due to source of inert fat and amount of basal fat in the diet.