Objective: The present study was carried out to evaluate the effects of probiotic in diets of growing calves on performance, blood metabolites and rumen metabolism parameters. Design: Descriptive study. Animals: Twelve Holstein calves (~151 kg) were divided randomly into three groups of four animals and were reared in clean well-ventilated boxes for 3 months. Procedure: In the treatment groups, probiotic (ANKOR ONE)® contained Saccharomyces cerevisiae, Aspergillus oryzae and Kluyveromyces marxianus was supplemented with concentrate mixture at the rate of 0.5 g/kg and 1 g/kg feed. All the calves were offered diet contained roughage and concentrate separately. Results: The average daily gain of the calves in the control group had significantly lower value (54 kg) as compared to those in the supplemented groups. Furthermore, calves fed unsupplemented diet had significantly the highest fecal score (3 = runny) compared to those fed supplemented diet with probiotic. Supplementation of probiotics in diets of calves did not show any significant differences in the blood glucose, total protein and albumin contents in comparison to those fed unsupplemented diet. Addition of probiotics led to significant higher concentrations of β-hydroxybutyrate and total volatile fatty acids of ruminal fluid in comparison to unsupplemented group. No significant effects for ammonia-N content of ruminal fluid were noted by feeding probiotic-supplemented diet compared to unsupplemented group. Conclusion and clinical relevance:  Probiotic supplementation in diets of calves generally improved average daily gain, reduced the incidence of diarrhea and did not adversely affect the levels of blood metabolites indices.