Abstract: The objective of this study was to investigate the effect of supplemental saturated fatty acids on blood CO₂, ions concentration and cation-anion balance of mid-lactating dairy cows under heat stress. Forty eight Chinese Holstein cows (150-210 days) were divided into three groups according to random block design with milk yield, DIM (days in milking) and parity. The animals were fed with diets contained 0, 1.5% and 3.0% saturated fatty acid for group 1(control), group 2(SFA1.5) and group 3(SFA3.0), respectively. The minimum THI (temperature-humidity index) was normally more than 72 during experimental period. Blood CO₂ concentration was 20.92, 21.82 and 21.31 mmol/L in cows fed with control, SFA1.5 and SFA3 diet, respectively. The blood CO₂ concentration was higher in SFA1.5 cows than control (P<0.05). There were no effects of treatment on ions concentration and cation-anion. The milk yield, milk fat content and total solid were improved by supplemental saturated fatty acid (P<0.05). In summary, supplemental saturated fatty acids improved milk yield and milk components and lessened blood CO₂ concentration decrease in mid-lactation heat-stressed dairy cows.