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The effects of tubule length and terminal capping on the geometrical and electronic properties of finite-sized zig-zag (9, 0) single-walled carbon nanotubes (SWNT), which length varying from 2 up to 12 unit cells (∼50Å), were investigated using molecular mechanics, semi-empirical methods (AM1 and EHMO) and density functional theory (B3LYP). AM1 method indicates how the nanotube ends are capped affects strongly the tubule geometric parameters. Although these effects seem to decrease exponentially as the tube gets longer, the converging values for C-C bond length in the open-and closed-end structures are slightly different. It was learned that combination of low-level methods like AM1 and EHMO (which tend to overestimate and underestimate the HOMO-LUMO energy gap, respectively) together with high-level method such as DFT is efficient to estimate band gap for finite-sized nanostructures. The HOMO-LUMO energy gaps obtained from semi-empirical and DFT methods decrease as the tubule length increases. Terminal capping also affects strongly the electronic structure of finite-sized nanotube. Thus, closing the terminal ends by fullerene hemisphere broadens the energy gap of the hydrogen-saturated open-end nanotube. Although the open-end SWNT has much lower AM1 HOMO-LUMO energy gap than the closed-end SWNT, these orbitals unfortunately are localized near the capping hydrogen, thereby do not provide conducting channels for electrons. By comparing only the delocalized frontier orbitals, both structures yield closer energy gap. Analysis of the energy gap based on EHMO, AM1 and DFT results suggests that both open-and closed-end finite-sized SWNT are semiconductor, in agreement with recent scanning tunneling experiment. It was found that the slight accumulated negative charges are likely to locate at the nanotube's fullerene tips.