Impurities in carbon nanotubes give rise to rich physics due to the honeycomb lattice structure. We concentrate on the conductance through a point-like defect in metallic zigzag carbon nanotube via the Landauer-B‥uttiker approach. At low bias, the conductance is suppressed due to the presence of an additional impurity state existing only on one of the sublattices. In consequence, the suppression is exactly half of the perfect conductance without impurity. Furthermore, there exists a transport resonance at larger bias where the perfect conductance is recovered as if the impurity were absent. Implications of these conductance
anomalies are elaborated and experimental detections in realistic carbon nanotubes are also discussed.

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