Brazil has a great variability of palm tree species with potential for vegetable oil production. Among them, babassu (Attalea speciosa Mart) and buriti (Mauritia flexuosa L.F.) are important species of the Amazon region. More than 75% of the fatty acids (FA) in babassu oil are saturated, especially in lauric acid (49%), while the buriti oil is primarily composed of oleic FA (76%). The rumen microorganisms are the main responsible for transform the unsaturated FA from feed in saturated and trans FA in ruminant products. The objective of this study was to evaluate the effects of adding babassu or buriti oil in the diet (DM basis) on biohydrogenation patterns in digestive contents of lambs. Twenty-one lambs (initial BW 18.9 ± 3.8 kg) were penned individually for 50 d and were distributed in a complete randomized block design. Animals were fed 1 of 3 diets: CONT (control diet containing 70% concentrate; compound by ground corn, soybean meal, mineral supplement and limestone) and 30% Tifton 85 hay; BAO (CONT plus 4% of babassu oil); or BUO (CONT plus 4% of buriti oil with ether extract content of 2.7, 5.9 and 6.0%, respectively). At the end of trial, lambs were slaughtered and samples of abomasal digesta contents were collected from each lamb. Samples were frozen, freeze-dried, milled and stored at −20°C until analysis by gas chromatography. Orthogonal contrasts were performed (CONT vs. diets containing oils and; BAO vs. BUO). BAO and BUO increased (P ≤ 0.05) ruminal biohydrogenation rate (BHR) of oleic acid (C18:1 cis-9) compared with the CONT diet (83.63, 93.25 and 89.64% for CONT, BAO and BUO, respectively). However, the addition of oils reduced (P ≤ 0.05) the BHR of linoleic acid (C18:2n-6) and linolenic acid (C18:3n-3). Additionally, BAO reduced (P ≤ 0.05) BHR of C18: n-3 compared with BUO (23.58 and 74.64% for BAO and BUO, respectively). BUO increased (P ≤ 0.05) the biohydrogenation completeness compared with BAO, but did not differ from CONT (93.92; 77.29 and 97.26% for CONT, BAO and BUO, respectively) because of higher concentration of unsaturated FA. The addition of babassu oil reduces the biohydrogenation completeness of fatty acids, increasing in this way the concentration of intermediate FA (trans) of the biohydrogenation process.