Abstract: In this paper, the electronic structure and optical properties of monolayer and three-layer MoS₂ are studied using first-principles calculations. The optical characters such as: dielectric function, optical reflectivity, and electron energy-loss spectrum of MoS₂ are observed in the energy region from 0 to 15 eV. To the density of states, the conduction band of three-layers of MoS₂ has only the low-energy part. At the equilibrium state, the dielectric constants of monolayer MoS₂ in the parallel Ex and Ez directions are 6.69 and 3.89, respectively, and that in the three-layer MoS₂ is 15.01 and 8.39, respectively. The dielectric function curve of monolayer MoS₂ has a fine structure than that of three-layer MoS₂.The dielectric functions have highly anisotropic in the energy range from 0 to 6 eV but they become isotropic when the energy is larger than 7 eV. In the Ex direction, monolayer and three-layer MoS₂ have an absorption peak in the visible region. Monolayer MoS₂ is more sensitive to near ultraviolet, while three-layer MoS₂ is more sensitive to far ultraviolet. As the number of layers increases, the refractive index increases. Both Monolayer and three-layer MoS₂ extinction coefficient curves have higher peaks in the near ultraviolet region than in the far ultraviolet area. It can be seen from the electron energy loss spectrum that with the increase in the number of layers, the peak position moves more and more to the high frequency area.