Using the first principles method, we have investigated the electronic properties of Si-based single-wallednanotubes with different diameters and chiral vectors. The electronic properties show significant differencewith those of carbon nanotubes. Si gearlike nanotubes g-NTs[1] are more stable according to the formationenergies, as Si atoms prefer the sp3 hybridization. Si n,n[1] n=5-11[1] g-NTs are semiconductors, whose gapsdecrease as the diameters increase. Si (n,0) (n=10-24) g-NTs are semiconductors and the gaps decrease in aperiod of 3. The results for large Si g-NTs can be explained using the tight-binding model and the method ofBrillouin zone foldings. The (n,0) (n=5-9) tubes are metal due to the α* and π* mixing, which is ratherstrong for the small tubes.