In this paper, the straight and T-shape single-walled carbon nanotube intramolecular junction (SWCNT-IMJ) were studied theoretically. The geometries of topological defect can be constructed by fusing two nanotubes having different helicities and diameters. For straight IMJ, the armchair (5,5) and (8,8) nanotubes segments were fused with the zigzag (n,0) segment varying from (6,0) to (10,0). T-junctions were constructed by joining a zigzag (n,0) tube, varying from n = 5 to 10, onto a fixed zigzag (9,0) tube sidewall. These junctions are composed of one or more pentagon and heptagon rings, and mixed pentagon-heptagon pairs as defects in the perfect hexagonal lattice. The study shows that the occurrence of defects geometries on SWCNT-IMJ and T-shape structures depends on the variation of tube helicity and diameter of combining nanotubes. The HOMO-LUMO energy levels exhibit the even-odd “quantum size” oscillation. The decreasing energy gap depends on variation of tube helicity and diameter of carbon nanotube. This study can be helpful as a knowledge base in the field of carbon nanotube molecular electronics.