Abstract: In order to determine the impact of muscle fibers on the load bearing capability of hydroxyapatites in vivo, hydroxyapatitie samples were implanted into the rat’s back muscles for different periods of the time. Then the compressive strength of the samples with attached muscles or soft tissues was measured in vivo. The strength of the implanted samples increases as muscles fibers growing into their micro pores. The compressive strength increases from 11.53 MPa before implantation to 14.84 MPa 20 days after implantation, then the increase is almost stopped 40, 60, 80 days after implantation. When the muscle fibers or soft tissues grow into the micro pores or are attached to the surfaces of hydroxyapatite samples, the strength of the surface layers is reinforced by the soft fibers, which perform as a composite shell wrapping over the inner hydroxyapatite and lead to the increase of strength. After the micro pores are full of fibers or the surfaces have been covered with soft tissues, new soft tissues will grow outside rather than into the micro pores and then reinforcing effect becomes weaker. This is the reason why the strength stops increasing after 20 days implantation.