In the present work, electro-optical and electronic properties of 4-n-pentyl-4'-cyanobiphenyl (5CB)and 4'-cyano-4-n-pentyl-p-terphenyl (5CT) liquid crystal molecules have been investigated. The 5CB compound transforms from crystal to nematic phase at 24°C and nematic to isotropic phase at 35.3°C while 5CT compound transforms from crystal to nematic phase at 130°C and nematic to isotropic phase at 239°C. Molecular geometry of 5CB and 5CT have been optimised by DFT method with 6-31G(d,p) basis set. Binding energies acting between dimers have been calculated. Mulliken and natural population analyses have been done. The HOMO–LUMO surfaces, molecular and thermodynamic properties have been examined. The excitation energy and oscillator strength have been calculated by TD-DFT/B3LYP/6-31G(d,p) method. All the investigated parameters of 5CT have been compared with that of 5CB molecule. Results have been used to elucidate the electro-optical and electronic properties of 5CB and 5CT molecules.