The thermal leptogenesis in brane world cosmology is studied. In brane world cosmology, the expansion law is modified from the four-dimensional standard cosmological one at high temperature regime in the early universe. As a result, the well-known upper bound on the lightest light neutrino mass induced by the condition for the out-of-equilibrium decay of the lightest heavy neutrino, m̃₁≲10⁻³  eV, can be moderated to be m̃₁≲10⁻³  eV × (M₁/Tₜ)² in the case of Tₜ≤M₁ with the lightest heavy neutrino mass (M₁) and the “transition temperature” (Tₜ), at which the modified expansion law in brane world cosmology is smoothly connecting with the standard one. This implies that the degenerate mass spectrum of the light neutrinos can be consistent with the thermal leptogenesis scenario. Furthermore, as recently pointed out, the gravitino problem in supersymmetric case can be solved if the transition temperature is low enough Tₜ≲10⁶⁻⁷  GeV. Therefore, even in the supersymmetric case, thermal leptogenesis scenario can be successfully realized in brane world cosmology.