In the present paper, a theoretical model is used to study the optical properties of nanomaterials. The optical properties of nanosized semiconductors are studied in relation to dimension and size. The model proposed by Guisbiers is extended to study the impact of size and shape on band-gap expansion, dielectric constant, vibrational frequency and electrical susceptibility of nanomaterials. The model is free from any adjustable parameter.We have considered here II–VI and III–V group semiconductors. It is found from model predictions that the energy bandgap of the nanosized semiconducting compounds increases as size decreases because of the quantum confinement of electrons and holes as size is reduced to nanolevel. The vibrational frequency, dielectric constant and electrical susceptibility are found to decreasewith decrease in the size of the nanosized semiconductors. The results calculated from the present model are found to be in good agreement with the available experimental and simulated data.