  Universal extra dimension models with Kaluza-Klein parity provide us
excellent candidates for dark matter. We consider phenomenological universal
extra dimension models where the Kaluza-Klein (KK) mass spectrum is different
from that of the minimal universal extra dimension model, and compute the
thermal relic abundance of the first KK mode of the photon taking into account
the production of second KK particles. It is pointed out that its thermal relic
abundance depends significantly on the mass degeneracy between the KK-photon
and other KK particles because of considerable coannihilation effects. The
cosmologically favored compactification scale is shown to range from around 1
TeV to a few TeV even in the cases where one of the first KK particles is
tightly degenerate with the first KK photon in mass.
