A total of 48 Manchego lambs were used in this growth trial and lambs were divided by diet and sex in four groups (12 lambs per group). Lambs were fed ad libitum and 2 kg/t of betaine was incorporated to the diet. No significant differences were found between diets when study the conformation and external fat of the carcasses. Significant differences (P < 0.05) were found for kidneys fat weight, with values of 165.42 g for control and 134.12 g for betaine. The same tendency (P < 0.01) was observed for kidneys fat cover, 2(+) was for control and 2(-) when betaine was added to the diet. A decrease in subcutaneous fat thickness was observed (P < 0.05) when betaine was added to the diet (2.54 vs 2.8 mm to betaine and control diet respectively). Betaine supplementation may have positive effect on animal production but this need to be further investigated.