The structural, electronic and optical properties of new YNi4Si-type RNi4Si (R = La and Gd) compounds are studied using full-potential augmented plane-wave method. To account better for the on-site f electron correlation, we adopted the Coulomb-corrected local spin density approximation (LSDA + U) exchange-correlation potential. Our optimized results of lattice parameters show good agreement to the previously reported experimental study. Analysis of the calculated band structure of LaNi4Si and GdNi4Si compounds demonstrates their metallic character. We found Ni-3d states mainly contribute to density of states from -5.0 eV to the Fermi level which is consistent with experiment and previously reported result by Kowalczyk et al. (J Magn Magn Mater 305:348-351, 2006), having hexagonal CaCu5 structure. Our calculated optical conductivity compares well with the experimental data and the results are analysed in the light of band-to-band transitions.