The dielectric functions of Ni3Al polycrystalline alloys were measured by spectroscopic ellipsometry in the energy range of 1.5–5.4 eV. The samples were Ni1−xAlx alloys with x=0.2625,0.2525,0.25,0.244, and 0.2359. One broad peak was observed in the optical-conductivity spectra around 4.4 eV. The band structures and the optical conductivity were calculated in the Au3Cu structure with the linearized-augmented-plane-wave method. By including both energy-dependent broadening and an approximate self-energy correction the calculated optical-conductivity spectrum gives quite good agreement with experiment. The calculation shows that the main contribution to the 4.4 eV peak comes from the k points close to the Γ-M-R plane and near the midpoints of the X-M line and Γ-M line.