Objective. In the present study the aim was to establish the efficacy of 30 mg/kg single dose secnidazol in calves naturally infected with Giardia duodenalis. Materials and methods. In an attempt to perform original study a total of 18 calves, from various breed, age and of both sexes were enrolled. Diagnosis was based on detection of trophozoit and/or cysts on fecal flotation among calves naturally infected with G. duodenalis, besides by use of rapid diagnostic test kits working with solid phase immunochromatographic principles and β-giardin nested-Polymerase Chain Reaction aplication. Cyst count per gram of feces were performed among days 0, 3, 7 and 10 in all cases. On days 0 and 10 hematological (WBC, RBC, HCT, MCHC, PLT) and serum biochemical (ALT, AST, creatinine, urea) values were determined. Results. Two different groups of calves composed of secnidazole group (n:9) and control group (n:9) were enrolled. Among calves enrolled in treatment group secnidazole was administered at a single dosage of 30 mg/kg perorally on day 0, whereas control group were left without any active ingredient. Cyst count per gram feces, hematological and serum biochemical values were analyzed among groups and intragroup comparisons. Giardia duodenalis assemblage A3 was detected in all 18 calves. On days 3, 7, and 10 there was significant (p˂0.001) reduction in cyst excretion; whereas evaluation of mean geometric cyst excretion revealed 100% reduction on days 7 and 10. Among two group statistical analysis of hematological and serumbiochemical variables revealed no statistical significance on days 0 and 10. Conclusions. In conclusion secnidazole at a single dose of 30 mg/kg might be practically appliable, reasonably priced, safety, completely effective and causing rapid recovery treatment protocole for therapy of calves with Giardiasis.