Light burned magnesia powders with different particle sizes were obtained by calcining crystalline magnesite. Sintered magnesia was prepared from the light burned magnesia powders by adding different amounts of La2O3. The effect of the La2O3 addition on the densification of magnesia was studied. The results show that the densification of sintered magnesia can be promoted both by reducing the particle size of the light burned powder and by adding small amounts of La2O3. The effect of the La2O3 addition is more significant: the maximum bulk density of the sintered magnesia can reach 3.49 g · cm^-3 and the apparent porosity can be reduced to 0.33%. During sintering, Ca²⁺ from the impurities enters La2O3 lattices, causing lattice distortion of La2O3 thus promoting the formation of La4.67(SiO4)3O phase and CaLa4(SiO4)3O phase, which fill the pores between the MgO grains and promote the densification of sintered magnesia.