The article presents the results of studying the effect of modified diatomite as part of a complex additive at the ratio of saturated and unsaturated fatty acids in cow’s milk. Production and physiological tests were carried out on the basis of a dairy farm LLC “Agrofirma Tetyushskoe” in the Ulyanovsk region on black-and-white cows of 2...3 lactation. For the physiological experiment, animals were selected by the method of analogues by live weight, number and month of lactation, pregnancy, and physiological state. The complex additive included modified diatomite, diacetophenonyl selenide containing organic selenium (DAFS), sunflower oil, and feed yeast. Additive based on 250 g/head/day mixed with feed once a day was administered in the diet of animals of the experimental group, the other group was a control and received no additive. The additive had an effect on the fatty acid composition of milk and contributed to an increase in the fraction of saturated fatty acids with a short chain. In the fat fraction, the amount of saturated short-chain fatty acids increases by 4.0% due to an increase in the content of capric (by 6.7%), lauric (by 12.3%, P<0.05), myristic (by 13.3%, P<0.05), palmitic (by 17.1%, P<0.05), which is a positive factor and indicates an active synthesis of volatile fatty acids, primarily acetate and 3-hydroxybutyrate. At the same time, there was a sharp decline in the content of stearic (27.3%, P<0.05) and arachinic (27.6%, P<0.05) acids. The milk of cows in the experimental group showed a decrease in the level of monounsaturated fatty acids by 7.4%.