In the present study, commercial purity Ti and Ti–6Al–4V alloy specimens with and without alkali- and heat-treatments were implanted in the abdominal connective tissue of mice. Conventional SUS 316L stainless steel was also implanted for comparison. After three months, their biocompatibility was evaluated by in vitro and in vivo experiments. When immersed in Hank’s solution, which has ion concentrations similar to that of human blood, an apatite layer, which accelerates the connection with bone, formed easily on the alkali-treated specimens compared to the non-treated specimens. The number of macrophages, which is known to increase as the inflammation reaction proceeds, is much lower for the alkali-treated specimens than for other samples. The average thickness of the fibrous capsule formed around the implant is much thinner for the alkali-treated specimens than for the others. These results show that the alkali- and heat-treatments performed in the present study will be very effective for the improvement of biocompatibility.