BACKGROUND: Natural feed supplements are gaining popularity in the animal production sector due to their safety and potential immunostimulatory properties, as well as the ban of some antibiotics and their negative residual effects. This study was carried out for 1 month to investigate the effect of Nannochloropsis oculata supplementation on growth performance and cell-mediated immunological status of rams assessed by leukogram assessment, lipid oxidation product malondialdehyde (MDA), total antioxidant capacity (TAC), interleukin assay after lymphocyte transformation test (LTT) including interleukin 6 (IL6), tumor necrosis factor-alpha (TNF-α), interleukin 12 (IL12), and gamma interferon (γ-IF), as well as Comet assay (% of DNA damage, tail length (px), % DNA in tail, tail moment and Olive tail moment). METHODS: Eighteen Barki rams (26.21 ± 0.64 kg) were divided into 3 equal treatment groups (6 sheep/each), G1: animals served as the control group that was fed the basal diet only, while the other treated groups (G2 and G3 (Nan 1.5% and Nan 3%) were fed the basal diet supplemented with 1.5% and 3% N. oculata (dry matter basis), respectively. RESULTS: The obtained results revealed that G3 showed a significant (P < 0.05) improvement in performance (body weight and body weight gain), the highest significant count (P < 0.05) in lymphocytes, and the lowest significant (P < 0.05) levels of neutrophils and neutrophils and lymphocytes ratio (N/L) ratio. Meanwhile, both levels of N. oculata significantly (P < 0.05) decreased MDA and increased TAC than control which seemed to be directly correlated with supplemented dose. There was a significant (P < 0.05) enhancement in the lymphocyte transformation assay produced significant (P < 0.05) high cytokines (IL6, γ-IF, IL12, and TNF-α) and the lowest significant (P <0.05) percent of DNA damage. The conducted principal component analysis estimated the inter-relationship between parameters and revealed that microalgae correlated strongly with cytokine assay and TAC, and negatively with Comet assay parameters; MDA, and neutrophils. CONCLUSIONS: It can be noted that dietary addition of N. oculata 3% increased sheep's performance while also producing significant-high cytokines. It also enhanced sheep immunology by considerably enhancing lymphocyte transformation ability. The antioxidant activity of Nannochloropsis appears to influence these findings. It was proposed that the Barki rams' basal diet be supplemented with 3% N. oculata.