Poultry genetics breeding to improve weight gain and reduce age of slaughter have been allowed the onset of cardiovascular system and leg problems. Leg disorders is the study of this work since it has been contributed to significant losses in poultry industry.  Femur of the modern broiler is the bone that must respond, not only changes in body weight with time, but also to changes in conformation (Lilburn et al, 2002). High dietary protein levels, high population density and lower light exposition have been contributed to legs abnormalities (Rath et al, 2000). Six hundred broilers, male and female, from three lines were reared in a wood shaving covered floor. The feeding program consisted of three diets: starter (1 to 21 days), grower (22 to 35 days) and finisher (36 to 42 days) diets formulated to satisfy their requirements in each phase. The experimental design was a completely randomized factorial of three lines and two sexes with 5 replicates with 10 birds each. Comparisons between means were carried out using Tukey’s test at 5% level. At 21, 28, 35 and 42 days of age, five male and five female/line were weighed, sacrificed and had their leg bones removed. The femurs were weighed and measured their length with a digital caliper (precision 0.02 mm) and frozen at -20°C. Femurs of the right legs were used to measure some physical and mechanical characteristics (density, porosity, cross-sectional area, moment of inertia, maximum load, strength, stiffness and toughness in bending of the bones) and partial chemical  composition (ash and ash Ca, P and Mg contents). It was observed a sexual dimorphism. Males grew at a faster rate than females and their bones were heavier, longer and had bigger cross-sectional area and femur flexural strength was highest in males than females. However, bone biochemical proprieties were not influenced by the growth rate or sex. Male bones had lower mechanical properties, which represents an intrinsic property of the bone, that can increase the risks to develop leg problems. Thus, the genetically improvement of weight gain without the correlation with sex and bone characteristics can result in increasing of the occurrence of structural bone problems.