The dielectric functions of Cu2ZnSnS4 bulk crystals grown by the Bridgman method were measured over the energy range 0.8 to 4.7 eV at room temperature using variable angle spectroscopic ellipsometry. The spectral dependence of complex refractive index, the absorption coefficient, and the normal-incidence reflectivity were also derived. The structure observed in the dielectric functions attributed to interband transitions E0, E1A E1B has been modelled using the Adachi’s model. The model parameters (strength, threshold energy, and broadening) have been determined using the simulated annealing (SA) algorithm. The results are in a good agreement with the experimental data over the entire range of photon energies.