This study was performed to investigate the effect of adding boron (B) to broilers' diets on performance, some blood biochemical parameters and visual and histopathological examinations of organs in broilers. One hundred and forty four chicks were randomly assigned to 4 groups with 3 replicates. The broilers were fed commercial diets supplemented with 0 (control group) and 500, 750, 1.000 mg kg(-1) (diet) B from Boric Acid (BA) for 42 days. Live body weight, weight gain, food consumption and feed conversion ratio were recorded at weekly intervals. At the end of the experimental period (42 days of age), serum Aspartate aminotransferase (AST), Alanine aminotransferase (ALT), Gamma Glutamyl Transpeptidase (GGT), Alkaline Phosphatase (ALP), Creatine Kinase (CK), Lactate Dehydrogenase (LDH) enzyme activities and glucose, creatinine, total protein, albumin, globulin, triglyceride, total cholesterol, High Density Lipoprotein-cholesterol (HDL-C), Low Density Lipoprotein-cholesterol (LDL-C), calcium (Ca), phosphorus (P), magnesium (Mg) and B levels were determined. Performance parameters were affected negatively and significantly by B supplementation. Serum AST, ALT, CK, LDH activities and glucose, total protein, albumin, globulin, triglycerides, total cholesterol, LDL-C and Mg levels were significantly influenced by dietary B supplementation. Necrosis in muscle fibers was observed but no gross and microscopic changes were determined in liver, kidney and brain in histological examinations in B supplemented groups. In conclusion; add boron to broilers' diets at levels 500, 750 and 1000 mg kg(-1) (feed) did not cause mortality or result in histopathological lesions except for muscles. On the other hand, it resulted in a significant decrease in performance and changed some biochemical parameters in broilers.