The major linear and nonlinear electric properties of urea and thiourea have been calculated at different levels of approximation with respect to the treatment of the electron correlation contribution. The basis set saturation effects were studied by comparing the results computed in three different basis sets of increasing flexibility. The so-called (first-order) polarized (Pol) basis have been found to give excellent results for dipole moments and dipole polarizabilities. Their performance in calculations of the first hyperpolarizability is also quite good. However, calculations of the second hyperpolarizability require the use of recently developed second-order polarized (HyPol) basis sets. The frequency-dependent dynamic polarizabilities and first and second hyperpolarizabilities of urea and thiourea have been calculated in the time-dependent Hartree-Fock approximation and then scaled to account for the electron correlation contribution and the basis set extension effect. This method gives useful estimates of the magnitude of different nonlinear optical processes. In particular thiourea was found to have attractively high value of the third-order optical susceptibility. The replacement of the C=O group of urea by its sulfur counterpart brings in general quite a large increase of the efficiency of the third-order optical processes.