In this Letter. the morphology, mechanical properties and electronic structure of single-walled carbon nanotube are investigated by use of a first-principles cluster method within the framework of local density approximation. The calculated C-C bond length, elastic modulus and tensile strength are in agreement with available experimental values. The C-C bond length is larger, and the elastic modulus and tensile strength are less in carbon nanotube than these in graphite. From the bonding analysis, we find that the bonding between carbon atoms in carbon nanotube is predominantly sigma bond, and pi bond takes an additional effect. Compared with graphite, the curvature effect of tube weakens the binding of a bond, but strengthens the binding of pi bond.