We consider a small extension of the standard model by adding two Majorana fermions; those are adjoint representations of the SU(2)L and SU(3)c gauge groups of the standard model. In this extension, the gauge coupling unification at an energy scale higher than 1015 GeV is realized when the masses of the triplet and the octet fermions are smaller than 104 GeV and 1012 GeV, respectively. We also show that an appropriate symmetry ensures a long lifetime of the neutral component of the triplet fermion whose thermal relic density naturally explains the observed dark matter density. The electron/positron excesses observed in recent cosmic-ray experiments can be also explained by the decay of the triplet fermion. Show Partial Abstract