The electronic structure and properties of FeS2 with the space groups of Pa3 and P1 were studied by the density functional theory. The generalized-gradient approximation exchange-correlation functional was used in conjunction with a plane wave-ultrasoft pseudopotential representation. Calculation results show that differences are observed in electronic structures and properties between Pa3 and P1 crystals. The band gap and energy loss of P1 are smaller than those of Pa3 crystal, while the dielectric constant, conductivity, refractive index, extinction coefficient, and intensity of optical absorption of P1 are larger than those of Pa3. These behaviors are attributed to the differences in symmetry, atomic arrangement, and Mulliken bond population of each unit for Pa3 and P1 crystals. The electronic structure and properties of FeS2 with the space groups of Pa3 and P1 were studied by the density functional theory. The generalized-gradient approximation exchange-correlation functional was used in conjunction with a plane wave-ultrasoft pseudopotential representation. Calculation results show that differences The band gap and energy loss of P1 are smaller than those of Pa3 crystal, while the dielectric constant, conductivity, refractive index, extinction coefficient, and intensity of optical absorption of P1 are larger than those of Pa3. These behaviors are attributed to the differences in symmetry, atomic arrangement, and Mulliken bond population of each unit for Pa3 and P1 crystals.