The structural, electronic, and optical properties of ZnSnO 3 were investigated using density functional theory within the generalized gradient approximation. The structure parameters obtained agree well with the experimental results. The electronic structures indicate that ZnSnO 3 is a semiconductor with a direct band gap of 1.0 eV. The calculated optical spectra can be assigned to contributions of the interband transitions from valence band O 2p levels to conduction band Sn 5s levels or higher conduction band Zn 3d levels in the low-energy region, and from O 2p to Sn 5p or Zn 4p conduction band in the high-energy region.