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Vitamin D is a metabolite which takes place especially in physiologic and metabolic events of poultry. In the broiler industry, applications are improving meat efficiency however the bone development is not improving respectively. This situation is the main reason of the long bones problems seen in broiler chickens after certain ages. The aim of this investigation was to study the effects of 25-OHD3 which is a form of vitamin D, on the bone development of broilers. Two hundred ten day-old meat-type chicks (308 Ross) were used. On the first day, the chicks were randomly separated in 7 groups. One of them was selected as the control group; all groups were included 30 chicks. The control groups were fed by standard commercial food. The other groups were fed by standard commercial food with three different doses of vitamin D 3 and two different doses of 25-OHD3 , metabolite of vitamin D3 which the commercial name is HyD. After the 42 days euthanasia was applied to the birds. Blood samples were taken at 21. and 42. days from hens. During the cutting process muscles were separated from the legs. The left and the right femurs and tibiatorsi were collected from 15 animals each of the groups and fixed in 10 percent formol saline solution. One of the femurs and tibiatorsi of each animal were used for bone strength tests. The others were kept in solution during 3 days and then they were transferred in to the 10 percent nitric acid solution for decalcification. The one femur and tibiatorsi of the decalcificated animals were removed and cut in half to prepare the transverse and sagittal sections. The routine processes were applied to those preparates and they were ambedded in parafin wax. 2 or 3 section of 4-5 micrometres were cutted from each parafin block and stained with hematocsylen eozine.The samples were investigated under the light microscope. In the microscopic evaluations, the criters indicating the growth plates, the bone density, the trabecules strength lesions were determined +1, +2, +3 in order to harmful to fine. The mean value of groups were compared to the control groups. The calcium and phosphor levels were determined spectrophotometrically. 25-OHD3 concentrations were determined with ELISA. The difference in 25-OHD3 levels between the control and the other groups was found statistically important significant. The differences between 2n d and 4t h groups and 5t h and 6t h groups were found statistically by microscopical evaluations. In the HyD groups, the bone strength tests results were found statistically important than the others. According to the results of microscopic observations and the bone strength tests, the two different dosages of HyD without adding Vit D3 do not contribute on bone development all by itself; however, it is concluded that the supplementation of 34 and especially the 69 mg HyD to the feeds which contain 2500 - 5000 IU/D3 effect the bone development and the strength.