This study was conducted to determine the effects of supplementing herbaceous peat preparation and vitamin C in order to find out its effects on immunity and growth performance in Holstein calves. Twenty Holstein calves (39.3±4.21kg average initial body weight) were allocated into four treatments by completely randomized design. Dietary treatments included: 1) control (Basal diet), 2) treatment 1 (control +5% herbaceous peat), 3) treatment 2 (control+10g vitamin C) and 4) treatment 3 (control+10g herbaceous peat+10g vitamin C). The duration of the experiment was 38 days in this experiment, and blood metabolism, dry matter intake, body weight and diarrhea frequency were investigated. For blood metabolism, white blood cells (WBC), neutrophils (NE), lymphocytes (LY), and red blood cells (RBC) did not show any significant differences among treatments. Average daily gain and feed conversion were highest in T1 group feeding with the herbaceous peat preparation compared to the control groups and diarrhea frequency showed lowest in T3 group. Overall results of the present experiment indicated that dietary supplementation with herbaceous peat influenced positively on growth performances in Holstein calves.