  Heavy fourth generation Majorana neutrino can be stable and contribute to a
small fraction of the relic density of dark matter (DM) in the Universe. Due to
its strong coupling to the standard model particles, it can be probed by the
current direct detection experiments even it is a subdominant component of the
whole halo DM. Assuming that it contributes to the same fraction of the local
halo DM density as that of the DM relic density, we show that the current
Xenon100 data constrain the mass of the stable Majorana neutrino to be greater
than the mass of the top quark. In the mass range between 200 GeV and a few
hundred GeV, the effective spin-independent cross section for the neutrino
elastic scattering off nucleon is insensitive to the neutrino mass and is
predicted to be $\sim 1.5\times 10^{-44} cm^2$, which can be reached by the
direct DM search experiments soon. In the same mass region the predicted
effective spin-dependent cross section for the heavy neutrino scattering off
proton is in the range $2\times 10^{-40} cm^2\sim 2\times 10^{-39} cm^2$, which
is within the reach of the ongoing IceCube experiment. We demonstrate such
properties in a fourth generation model with the stability of the fourth
Majorana neutrino protected by an additional generation-dependent U(1) gauge
symmetry.
