Bone is a composite material consisting of biopolymer (type-I collagen) and a mineral phase (carbonated hydroxyapatite). Viscoelastic properties of fully demineralized cortical bovine femur bone (collagen) were investigated by loading-unloading compression testing in three anatomical directions (longitudinal, radial, and transverse). Anisotropic behavior was observed: the radial and transverse directions were found to be more energy absorbent, while the longitudinal one was found to be the stiffest. A possible explanation for these phenomena is attributed to the difference in collagen fibers alignment in all three bone anatomical directions. Additionally, stress relaxation experiments were performed for all three anatomical directions under two different conditions (2% and 5% loading strain that correspond to naturally appeared stains in bone collagen physiological region). Several step-wise regions shown to correspond to the collagen microstructural features were observed on the relaxation curves for samples in the radial and transverse directions. Data analysis shows that the collagen relaxed rapidly to zero for the radial and transverse directions, while it showed a slower, more stable exponential decay for the longitudinal direction. These data support the idea that not only does bone as a composite material exhibit anisotropic mechanical properties, but its major constituent also shows such behavior. This research is funded by the National Science Foundation, Division of Materials Research, Ceramics Program (Grant 1006931).