Electrical conductivity and oxygen permeation rates in Co-doped Sr3Ti2O7-delta with Ruddesden-Popper type structures were investigated. The effects of metal dopants (M) in the Ti site of Sr3Ti2(M)O7-delta on the mixed conductivity were also studied. Doping of Sr3Ti2O7-delta with Co was found to be effective for improving the electrical conductivity as well as the oxygen permeation rate, which could be assigned to the increased oxygen vacancy concentration by doping Co3+ into Ti4+ sites. The nonstoichiometric oxygen of these oxides was measured by using a thermal gravimetric method. The creation of oxygen vacancies, which is compensated with Co3+ doping, leads to higher oxide ion conductivity. The oxygen permeation rate monotonously increased with increasing amounts of Co in the Ti site. Sr3Ti0.8Co1.2O7-delta exhibited high oxide ion conductivity and a large oxygen permeation rate. The highest oxygen permeation rate achieved a value of 2.02ccmin-1cm-2 at 1273K for Sr3Ti0.8Co1.2O7-delta. Neutron diffraction analysis and redox titration suggests that the oxygen diffusion occurs through oxygen vacancies in the perovskite block, but not through excess oxygen in the rock salt block.