  Relic abundance of dark matter is investigated in the framework of universal
extra dimension models with right-handed neutrinos. These models are free from
the serious Kaluza-Klein (KK) graviton problem that the original universal
extra dimension model has. The first KK particle of the right-handed neutrino
is a candidate for dark matter in this framework, and its relic abundance is
determined by three processes, (1) the decay of the KK photon into the first KK
right-handed neutrino in the late universe, (2) production of the first KK
right-handed neutrino from the thermal bath in the early universe, and (3) the
decay of higher KK right-handed neutrinos into the first KK right-handed
neutrino in the late universe. When ordinary neutrino masses are large enough
such as the degenerate mass spectrum case, the last process contribute to the
abundance significantly, even if the reheating temperature is low. The scale of
the extra dimension consistent with cosmological observations can be 500 GeV in
the minimal setup of universal extra dimension models with right-handed
neutrinos.
