Dairy cows fitted with ruminal, duodenal and ileal cannulas were utilized to investigate the effects of feeding with Ca soaps (CaS) of palm fatty acids (FA) and rapeseed FA. Diets compared were control diet based on maize silage and concentrate, and two diets with 40 g CaS of palm oil FA or rapeseed oil FA/kg diet, replacing part of the concentrates of the control diet. Total digestibilities of dry matter, fibre and fat, and ruminal fermentation were not significantly altered by giving CaS; the extent of ruminal biohydrogenation of total unsaturated C18 FA was significantly reduced by both CaS diets. Apparent intestinal digestibility of FA was not different among diets, although the amount of FA absorbed with the CaS diets was twice that with the control diet. No difference among diets was observed for milk production, or fat and protein contents. Giving CaS diets decreased the proportions of 4[ratio ]0 to 14[ratio ]0 FA in milk fat, and increased cis-18[ratio ]1n−9, compared with control diet. The rapeseed diet lowered the content of 16[ratio ]0, and increased the contents of 18[ratio ]0 and trans-18[ratio ]1n−7. CaS diets did not result in a marked increase of polyunsaturated FA content in milk fat. Butter from cows fed on the CaS diets contained more liquid fat at 6 and 14°C than butter from the cows fed on the control diet. Incorporating CaS, particularly those from rapeseed, in dairy cows' diets increased C18 FA in milk and improved butter spreadability.