The nonunion fracture is a relatively frequent complication in both human and veterinary medicine. Specifically, atrophic fracture nonunions are difficult to treat, with revision surgery usually providing the best prognosis. Anabolic steroids, such as nandrolone decanoate (ND), have been reported to have beneficial clinical effects on bone mass gain during osteoporosis; however, their utility in promoting regeneration in atrophic nonunions has not been documented. Our objective was to examine morphological changes induced by the ND in experimental fracture nonunion with vascular deficit in the rat model. Fourteen adult Wistar rats had an atrophic fracture nonunion induced in the diaphysis of their left femur. Rats were allocated into two groups: control group and nandrolone decanoate group. Rats in the latter group were given nandrolone decanoate (1.5 mg/kg IM, once a week, during 4 weeks after confirmation of fracture nonunion radiographically). Radiographic and anatomopathological examination, micro-tomography and histological analysis were assessed to characterize the morphological changes promoted by the nandrolone decanoate use. Based on radiology, anatomopathological evaluation, computed micro-tomography and conventional microscopy, nandrolone decanoate promoted bone regeneration at the fracture nonunion site by increasing the cellularity at the fracture site. Percentage of collagen was not significantly different between groups, consistent with high-quality regenerated bone. The anabolic steroid nandrolone decanoate improved bone mass and regeneration without affecting collagen production and therefore has potential for improving outcomes for atrophic fracture nonunion.