Electron and potential distributions in metallic carbon nanotubes (CNTs) under applied voltages are simulated with fully self-consistent real and mode space non-equilibrium Greenpsilas function (NEGF) and tight-binding (TB) method I-V characteristics are also obtained. It is shown that mode-space method is a good approximation to real-space method and could significantly reduce computational cost. A rough relation is given to determine whether to include a specific mode in simulation or not. Conductance of small-diameter CNTs is found to coincide with theoretically calculated conductance of a nano-scaled conductor in ballistic limit. Computed conductance of large-diameter CNTs is large than 4e2 / h due to contribution of upper subbands.