The optoelectronic properties of KDP (KH2PO4), in orthorhombic and tetragonal phases, have been investigated by generalized gradient approximation (GGA) and mBJ-GGA within density functional theory. The calculated fundamental indirect band gaps for orthorhombic and tetragonal KDPs are 2.83 and 4.35 eV, respectively. The calculated effective mass of electron, in this compound, is very close to the effective mass of carriers in a bilayer graphene. The electron density plots show that the covalent bonds between P and O within the PO4 tetrahedra are symmetric in tetragonal phase, whereas in the orthorhombic phase this symmetry breaks, which affects the covalent bonds among the tetrahedra. The comparison of the calculated optical spectra by GGA and mBJ-GGA with the experimental results reveals that the optical dispersion spectra by mBJ-GGA are in closer agreement with the experimental results. © 2012 Wiley Periodicals, Inc.