A Matlab program was self-developed for simple Hückel molecular orbital calculations, with which carbon nanotubes of CNT(5,5) doped with vacancy, Stone-Wales defect, N and B atoms were analyzed. Calculated π-electron density and frontier molecular orbitals (HOMO and LUMO) provide a basis to study chemical reactivity of CNTs. Homogenous distributions of π-electron, HOMO and LUMO are broken by dopants with different electronic character. Furthermore, dopants and/or nearby carbon atoms contribute more in HOMO or LUMO, behaving nucleophile or electrophile in different reactions, respectively. Calculated HOMO-LUMO gaps reflect electrical conductivity of doped CNTs well. Calculated results are in agreement with experimental and theoretical results reported elsewhere.