The pseudopotential method has been used to investigate the structural, electronic and magnetic properties of La1-xEuxGaO3 （x = 0, 0.25, 0.5, 0.75, and 1） within the scheme of generalized gradient approximation. The spin-polarized calculations demonstrate that the ground state is an antiferromagnetic insulator for x ≤ 0.5, while it is ferromagnetic halfmetal at x 〉 0.5. The substitutions of magnetic Eu ions for non-magnetic La ions produce and strength spin polarization, which forcefully urges the system from the insulator to the half metal. Meanwhile, Eu doping strengthens a stoner mechanism for ferromagnetism of La1-xEuxGaO3 （x = 0.75 and 1）, which may lead to a rapid increasing in the total magnetic moment and therefore, antiferromagnetic-ferromagnetic transition happens.
