In this study, Yucca schidigera powder (YSP) added to lamb feed was investigated for its effects on performance, some blood parameters, the immune system, and the antioxidative metabolism of the hepatic tissue. Throughout a 10-week fattening period, twenty-four 2.5-month-old male Akkaraman lambs were used. Three groups were established, including a control group (C) and groups YSPI and YSPII, which received a basal ration alone, a basal ration + 100 ppm YSP, and a basal ration + 200 ppm YSP, respectively. The groups did not differ for performance parameters (P > 0.05). While 100 ppm YSP increased serum NH3 levels (P < 0.05), 200 ppm YSP significantly decreased serum cholesterol, HDL, LDL, and Mg levels (P < 0.05). The groups did not differ for antibody titers against the Brucella melitensis vaccine (Rev 1). In groups YSPI and YSPII, hepatic CAT activity and LPO levels were significantly lower than those of Group C (P < 0.01). While the GSH levels of group YSPI were significantly higher than those of groups C and YSPII, the SOD activity of group YSPII was significantly lower than those of groups C and YSPI (P < 0.01). As a result, while YSP did not affect the performance parameters or immune system, it significantly affected NH3 levels, lipid metabolism, and the antioxidant system.