Based on the tight-binding model, we develop a transfer-matrix method to study the electronic conductance of single-walled carbon nanotubes. It is found that there exists a narrow conductance gap in metallic zigzag nanotubes corresponding to the curvature-induced energy gap. In addition, the conductance near the Fermi level depends on both the diameter and the length of the carbon nanotube. Moreover, ballistic, diffused or classical transport may be found in carbon nanotubes with variation of energy.