La modified Pb(Mg1/3Nb2/3)O-3-PbTiO3 (PLMNT) transparent electro-optic (EO) ceramics with pure perovskite phase were fabricated by the uniaxial hot-press sintering method. An improved hot-press sintering technique with varied atmosphere was proposed to eliminate the pore defects. It consists of four stages: 1) sintering at a relatively low temperature (800 degrees C) in the vacuum atmosphere first, and then in the pure O-2 atmosphere; 2) sintering in a pure O-2 atmosphere at high temperature by uniaxial hot-press method; 3) replacing the O-2 atmosphere with CO2 gas to eliminate the residual closed pores; 4) finally exchanging CO2 atmosphere with pure O-2 gas to eliminate the oxygen vacancies. The research results certified that the thermal treatment in the vacuum atmosphere and CO2 atmosphere could eliminate the residual closed pores and improve the transmittance of the PLMNT transparent ceramics effectively. By the improved hot-press sintering technique, the optical transmittance of the polished PLMNT transparent ceramics with a thickness of 0.35mm increases from 61% to 64% at 632.8nm. The reasonable explanation was given about the mechanism of the improvements of this sintering technique.