Abstarct: In this thesis, physical properties of pure and doped GaFeO3 were investigated using OpenMx computational package baxsed on density functional theory in GGA+U approximation. At the first step, the structural, electronic and magnetic properties of GaFeO3 were calculated with both GGA and GGA+U approximations. The results showed that GaFeO3 has orthorhombic structure with Pc21n symmetry and A-type antiferromagnetic spin configuration in the ground state. The antiferromagnetic property arises from the indirect exchange interactions via the Fe1-O-Fe2 bonds. Also, calculations indicated that GaFeO3 is semiconductor with band gap of 2.44 eV in both spin up and spin down channels. Distortion of Fe atoms from the center of relevant octahedrons leads to the symmetry breaking and is considered as an origin of ferroelectric property. In the second step, by substitution of Ga with Zn in GaFeO3 compound (Ga1-xZnxFeO3, x = 12.5%, 25%, 37.5%, 50%) the effect of Zn impurity on the structural, magnetic and electronic properties of GaFeO3 was investigated. Samples with the specified concentrations were named GZFO1, GZFO2, GZFO3, and GZFO4, respectively. The structural studies showed that the lattice constants, orthorhombic distortion and the cell volume increase upon Zn substitution. The magnetization of the Zn-substituted samples is enhanced compared with that of the pristine sample of GFO. Also, a magnetic phase transition from antiferromagnetic to ferrimagnetic is observed. The electronic investigations revealed that GZFO1, GZFO2 and GZFO3 samples are half-mextallic compounds while GZFO4 sample is a mextallic compound. In the next step, by substitution of Fe with Mn in GaFeO3 compound (GaFe1-xMnxO3, x = 12.5%, 25% and 37.5%) the effect of Mn impurity on the structural, magnetic, electronic and optical properties of GaFeO3 was investigated. Iron plays a significant role in the antiferromagnetic super-exchange mechanism. Substitution of Fe with Mn atom could change the antiferromagnetic super exchange interaction in the Fe1–O–Fe2 bond of GFO, and as a result alters the structural, electronic and magnetic properties of GFO. Structural investigations showed that the lattice parameters and the unit cell volume are increased with Mn substitution. Also, due to the difference in magnetic moments of Fe and Mn atoms, magnetization of the Mn-doped samples enhances as the Mn concentration increases and a magnetic phase transition from antiferromagnetic to ferrimagnetic is occurred. Electronic investigations revealed the formation of new states around the Fermi surface with Mn substitution in the spin up channel. These new states reduce the band gap in spin up channel and facilitate optical transitions in energies less than the gallium ferrite band gap.