The effects of two thermal treatments of soybeans, i.e. roasting (150˚C dry heat) and extrusion (140-150˚C), on conjugated linoleic acids (CLA) and trans-octadecenoic acids (trans-C18:1) productions obtained throughout ruminal C18:2 biohydrogenation in cows were examined. Nylon bags containing raw, roasted or extruded soybeans were incubated in the rumen of dry fistulated cows, during 2, 4, 8, 16 or 24 hours. After incubation of 2-4 h, significantly greater amounts of linoleic acid (C18:2) remained in bags containing extruded and roasted soybeans than in those with raw soybeans, reflecting a lower biohydrogenation of C18:2 in both case. Furthermore, significant and marked accumulations of CLA and trans-C18:1 at a lesser extend were noticed in bags containing extruded soybeans compared to those with raw or roasted soybeans. By calculations of the efficiencies of the three reactions, an inhibition of the C18:2 isomerisation was evidenced with extruded and roasted soybeans, as well as an inhibition of the two reduction steps in presence of extruded soybeans. Consequently, the thermal treatment and the nature of heating process of fat are efficient ways to modulate the CLA and trans-C18:1 ruminal productions.