This study investigated the effects of tea saponins (TSP) on milk performance, milk fatty acids, and blood immune function in dairy cows. A total of 20 early-lactation Holstein cows (day in milk = 66.4 ± 16.8 d; parity = 1.75 ± 0.91; and milk yield = 36.3 ± 7.32 kg/d; mean ± SD) were randomly divided into 4 homogeneous groups, with TSP added at the levels of 0 (control), 20, 30, and 40 g/d per head, respectively. All cows had 2 weeks of adaptation and 6 weeks of treatments. Feed, milk, and blood were sampled and analyzed weekly. The data were analyzed using PROC MIXED program of SAS with a randomized complete block design with repeated measures. The yield of milk, milk protein, lactose, and total solids were lower in the cows fed 40 g/d TSP compared with other groups (P < 0.01). Energy-corrected milk of cows fed 30 g/d TSP was greater than the control (P < 0.01). The milk fat content of cows fed 40 g/d TSP was greater than the control and the 20 g/d TSP group. The proportion of milk unsaturated fatty acid such as cis-9 C16:1, cis-9 C18:1, and C20:4n-6 in the 40 g/d TSP group was greater than in the control (P < 0.05). Plasma malondialdehyde concentration decreased as the supplemental value of TSP increased (quadratic P < 0.01), whereas, the concentration of superoxide dismutase increased as the supplemental value of TSP increased (linear P < 0.01). The plasma concentration of tumor necrosis factor-α and interferon-γ increased as the supplemental value of TSP increased (linear P < 0.01). This study showed that the supplementation of 40 g/d TSP to dairy cows could decrease the dietary dry matter intake, resulting in a lower milk yield. The supplementation of TSP could reduce oxidative stress in the cows and might improve their immunoreaction. The increased milk fat content and milk unsaturated fatty acids proportion were achieved in cows fed 40 g/d TSP, which may indicate the TSP played a part in lipometabolism of dairy cows.