The influences of the post-perovskite (PPV) phase transition on the thermal state in the lower mantle are studied with a three-dimensional model of mantle convection in a Cartesian domain under the extended Boussinesq approximation with variable viscosity and temperature-dependent thermal conductivity. We have varied (i) the intensity of latent heat exchange associated with the PPV transition by increasing the density change and (ii) the temperature at the core-mantle boundary (CMB) which determines the stability field of the PPV phase through the relative positioning with the phase transition temperature at the CMB. We found that the actual PPV transition hardly affects the thermal structure in the lower mantle, although it has a tendency to bend the vertical temperature profile toward the equilibrium thermodynamic conditions of the transition in extreme cases with enhanced latent heat exchange and CMB temperature lower than that of phase transition.