Ignition characteristics in deuterium-tritium (D-T) and D-3He tokamak reactors with spin-polarized fuels are presented by using the ignition access condition based on the generalized saddle point in the representation of . Enhancement of the D-T fusion cross section due to parallel spin polarization with respect to the magnetic field can reduce the confinement enhancement factor required for reaching ignition by ∼20% if fusion particle loss is not induced by the anisotropic fusion particle distribution. Spin polarization is thus effective when a D-T reactor is marginal for ignition. In D-3He fusion, it is more advantageous to use spin-polarized fuel in the heating phase than in the case of D-T fusion. The ignition toroidal beta value can be reduced by spin polarization from 12 ± 0.8 to 5.3 ± 0.5% in D-3He = 2:1 plasma and from 17 ± 0.5 to 6.5 ± 0.2% in D-3He = 1:1 plasma. The auxiliary heating power to reach ignition, which is rather large for D-3He fusion, can be reduced by a factor of 2 to 3 compared with the unpolarized case. For example, in the D-3He Tokamak Reactor, 350 MW of auxiliary heating power for D:3He = 2:1 and Ti(0)/Te(0) = 1 without spin polarization can be reduced to 190 MW with complete polarization of the deuterium and 3He ions. The deuterium-deuterium fusion suppression effect, if it exists, does not alter the ignition condition much. Various problems related to the spin polarization scheme are also discussed.