A theoretical study of electronic and optical properties of zinc blende structure of BeS is presented by applying the full-potential linearized augmented plane wave (FP-LAPW) method within density-functional theory (DFT) as implemented in WIEN2k code. Our results are in agreement with previous theoretical and experimental studies on this important semiconductor compound. For the optical calculations, the dipole approximation is used. The imaginary part of dielectric function is calculated in momentum representation, which requires matrix elements of the momentum p between occupied and unoccupied states. The microscopic origin of the main features in the optical spectra is identified and also the factors responsible for most of the optical absorption.