• Stearic acid (C18:0) reduces the performance of late lactating dairy ewes. • C18:0 reduces gene expression of genes from the de novo pathway synthesis. • C18:0 did not increase milk fat content in late lactating dairy ewes. The objective of this study was to evaluate stearic acid supplementation (C18:0) on the production, composition, fatty acid profile and the expression of lipogenic genes in the mammary gland of late lactating ewes. We used 30 primiparous and multiparous Lacaune ewes with a mean body weight (BW) of 66.5 ± 9.4 kg and body condition score (BCS) of 3.0 ± 0.5 at late lactation (122 ± 12 DEL), producing 1.0 ± 0.3 kg of milk/day. The treatments were: Control and C18 (28 g/animal/d of C18:0). When compared with Control, C18:0 reduced silage dry matter intake (DMI) by 13.1 % (P = 0.0003), milk production by 8.1 % (P = 0.05), lactose production by 3.3 % (P = 0.0004), and lactose content by 3.4 % (P = 0.0002). There was no treatment effect on the production and content of fat, protein, and total solids. The gene expression of acetyl-CoA carboxylase alpha promoter 2 (ACACA PII) and fatty acid synthase (FASN) were reduced by 30 % (P = 0.02) and 26.1 % (P = 0.04), respectively. The C18:0 supplementation is not effective in increasing the fat content in late lactating dairy ewes and reduces DMI, milk production, lactose levels, and gene expression involved in fatty acid synthesis.