The relaxation Young's modulus, ε(t), of bovine femoral bone was measured as a function of mineral content. Five different specimens with different mineral contents were prepared by the EDTA treatment. The relaxation curves for specimens of different mineral contents were superimposable upon one another by shifting along the log t axis as well as the log ε(t) axis. A well-specified master curve of stress relaxation was obtained from a set of relaxation curves for samples with different mineral contents. The mineral content dependence of vertical shift factors, along the log ε(t) axis, accorded well with the mineral content dependence of Young's modulus itself, the accordance indicating the plausibility of the vertical shifting procedure. The stress relaxation in bone has been reported to be related to the viscoelastic properties of the collagen matrix. It is considered that the reinforcement of the matrix around the mineral, by the mineral as fillers, increases the average modulus of the matrix and lengthens the characteristic time of the relaxation. This consideration explains the existence of a shift factor along the logt axis and its mineral content dependence. As the structures of bone specimens treated by EDTA are expected to be different from that of normal bone, the conclusion drawn from this experiment cannot be immediately applied to normal bone. It can be, however, concluded that the size of the mineral and also the interface area between mineral and collagen matrix play an important role in the viscoelastic properties of normal bone and the bone speciment treated by EDTA.