The spin-dependent electron transport properties of a metallic (5,5) single-walled carbon nanotube with either carbon (C) or boron (B) adatoms were investigated using a non-equilibrium Green's function technique combined with spin-dependent density functional theory. We find that both of the non-magnetic B and C adatoms cause zero-bias conductance that is highly dependent on the spin states of the conduction electrons. The microscopic origin of this phenomenon is explained by the features of

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... spin-dependent local density of states in the region of the adatom. From the present calculation it was also determined that the spin-dependent conductance is controllable by tuning the applied gate voltage, which would be a useful property for application in spin filters.