Bone is composed of a mineral matrix reinforced by a network of collagen that governs the biomechanical functions of the skeletal system in the body. The purpose of the study was to investigate the possible effect of extremely low-frequency magnetic field (ELF-MF) on geometric and biomechanical properties of rats' bone. In this study, 30 male Sprague-Dawley rats were used. The rats were divided into three groups: two experimental and one control sham. The first and second experimental group (n = 10) were exposed to 100 mu T and 500 mu T-MF during 10 months, 2 h a day, respectively, and the third (sham) (n = 10) group was treated like experimental group except ELF-MF exposure in methacrylate boxes. After ELF-MF and sham exposure, geometric and the biomechanical properties of rats' bone, such as cross-sectional area of the femoral shaft, length of the femur, cortical thickness of the femur, ultimate tensile strength (maximum load), displacement, stiffness, energy absorption capacity, elastic modulus, and toughness of bone were determined. The geometric and biomechanical analyses showed that a significant decrease in rats exposed to 100 mu T-MF in comparison to sham and 500 mu T-MF exposed rats about the values of cross-sectional area of the femoral shaft (P < 0.05). Maximum load increased in 100 mu T-MF and 500 mu T-MF exposed rats when compared to that of the sham rats (P < 0.05). The cortical thickness of the femurs of MF-exposed rats (100 mT and 500 mT) were significantly decreased in comparison to that of sham groups' rats (P < 0.05 and P < 0.001). However, no significant differences were found in the other biomechanical endpoints between each other groups, such as: length of the femur, displacement, stiffness, energy absorption capacity, elastic modulus, and toughness of bone (P > 0.05). These experiments demonstrated that 100 mu T-MF and 500 mu T-MF can affect biomechanical and geometrical properties of rats' bone.