This study aimed to evaluate the effects of Essential oil (EO) from Arnica montana as a feed additive on feedlot performance, ingestive behavior, carcass characteristics, rumen morphometrics characteristics and meat fatty acids profile of lambs. Forty Dorper × Santa Inês male lambs with 30.3 ± 5.76 kg (mean ± standard deviation) of initial body weight and 92 ± 9 days (mean ± standard deviation) of age were used. The experimental design was in a randomized complete block (5 treatments and 8 blocks). Lambs were fed a basal diet (160 ± 13 g/kg DM of CP; mean ± standard deviation) containing 90 % concentrate and 10 % forage (Coastcross hay). The experimental diets were: A0 - negative control (basal diet without additive); A450 - addition of 450 mg/kg DM of EO from Arnica montana; A900 - addition of 900 mg/kg DM of EO from Arnica montana; A1350 - addition of 1350 mg/kg DM of EO from Arnica montana; M25 - positive control (basal diet containing 25 mg/kg DM of sodium monensin). Increasing EO from Arnica montana content conferred a quadratic effect on dry matter intake (DMI) (P = 0.02), with higher DM in A450. The DMI was higher (P < 0.01) in diets with EO from Arnica montana compared to M25. The average daily gain (ADG), body weight at slaughter (BWS), hot and cold carcass weight (HCW and CCW) increased linearly with increasing EO from Arnica montana (P < 0.05). The lambs had higher ADG, BWS, HCW and CCW with EO from Arnica montana than M25 (P < 0.05). The oocyst/g feces decreased linearly (P < 0.0001) with increasing EO from Arnica montana in the diet. However, the M25 was more efficient in reducing the oocyst than the other treatments (P < 0.05). The number of meals and time spent feeding increased linearly (P < 0.05) with the increase of EO from Arnica montana. There was a quadratic effect (P < 0.05) on feeding rate (min/g DM), with the lowest rate in A450. Time spent ruminating (min/day) was affected quadratically (P = 0.01) by increasing EO from Arnica montana, this activity was higher in A900. Compared to M25, time spent ruminating and total chewing time were longer (P < 0.05) and the feeding rate and chewing rate (min/g of NDF) were lower (P < 0.05) in diets with EO from Arnica montana. The higher inclusion of EO from Arnica montana decreased linearly the C17:0; C18:0; C18:1 trans-11, C22:6 n3 and the sum of saturated fatty acids (ΣSFA), and linearly increased the C18: 2 cis-9, cis-12; C18:3 cis-9, cis-12, cis-15, the sum of polyunsaturated (ΣPUFA) and the sum of n6 (P < 0.05). Diets with EO from Arnica montana decreased the C16:0, C17:0, C18:0 and increased the C18:2 cis-9, cis-12, ΣSFA, Σn6 and the n6/n3 ratio (P < 0.05) compared to M25. Sodium monensin at a dose of 25 mg/kg DM can be replaced by EO from Arnica montana in high-concentrate diets to promote lamb performance, carcass characteristics and fatty acid profile in the meat. The use of 1350 mg/kg DM of EO from Arnica montana was the best dose in high-concentrate diets in feedlot lambs, due to positive effect on performance, carcass characteristics, and the fatty acid profile of the meat.