The aim of this experiment was to study the effect of Conjugated Linoleic Acid (CLA) supplement on milk yield, milk composition,energy balance and blood metabolites of Holstein cows in their early lactation. Fifteen multiparous lactating Holstein cows (20±3 DIM, 637±8 kg initial BW) were blocked by milk production, and assigned in a completely randomized block design to 1 of 3 treatments, namely: 1) control (basal diet+115g/d Ca salts of palm fatty acids), 2) Rumen protected CLA (basal diet+120g/d of lipid encapsulated CLA), and 3) Rumen unprotected CLA (basal diet+40g/d rumen unprotected CLA). Doses provided a total of 96g of fatty acids per day. Dry matter intake and milk yield were recorded daily. Milk composition, body weight and body condition score were measured weekly during the 35 days of the experiment. Blood samples were collected twice in days 14 and 35 of experiment. Results indicated that rumen protected CLA supplement reduced milk fat content by 17.8% as compared with control (P<0.01) but it had no effect on milk protein and lactose content. There were no differences in dry matter intake and body condition score. Milk production was 41.7, 44.6 and 42.0 kg for control group, rumen protected CLA and rumen unprotected CLA respectively. There was a significant increase in milk production in rumen protected CLA group as compared to others (P<0.01). Amount of milk energy output and energy balance were not affected by the treatments. Plasma concentrations of glucose, NEFA, BHBA and blood urea nitrogen were similar among treatment groups. The results of this study indicate that 120 g/d of rumen protected CLA could increase milk production and decrease milk fat content in early lactation but, had no significant effect on milk protein and lactose content and on energy balance.