In this study, supplementation of two levels (5 and 25 parts per million; ppm) of boron into broiler diets including 125 IU kg-1 (inadequate) and 2000 IU kg-1 (adequate) vitamin D3 as investigated. The effects of supplementation on performance and biochemical characters (Ca, P, Mg, glucose and AP) of broilers from 1 to 45 days of age were evaluated. Boron provided significant increases in performances of chicks fed both adequate and inadequate vitamin D3-containing diets. The improvements in the inadequate vitamin D3-containing group were higher than that of adequate vitamin D3-containing group. The boron addition had a positive effect on Ca, P and alkaline phosphatase levels of chicks. Boron might be regarded as beneficial in inadequate vitamin D3-containing broiler feed. © 2001 Harcourt Publishers Ltd.