The work aims to study the effect of the content of rare earth oxide La2O3 in the boronizing agent on the thickness and properties of boronizing layer of 45 steel. 0% La2O3, 5% La2O3 and 10% La2O3 were added to boronizing agent for boronizing 45 steel at 850 ℃ for 4 hours. The morphology and thickness of the boronizing layer were observed and measured by metallographic microscope; the phase structure of boronizing layer was analyzed by XRD; and the hardness, wear resistance, corrosion resistance and brittleness of boronizing layer were measured and determined by microhardness tester, wear tester and electrochemical workstation. Boronizing layer of 45 steel could remarkably improve the hardness of base material, and adding 5% La2O3 and 10% La2O3 to the infiltration agent could significantly increase the thickness and hardness of boronizing layer compared with that without La2O3 in the infiltration agent. However, different contents of La2O3 had different effects on the wear resistance, corrosion resistance and brittleness of boronizing layer. The boronizing layer with 5% La2O3 in the infiltration agent had the best wear resistance and corrosion resistance and the lowest brittleness. The wear resistance and brittleness of boronizing layer with 10% La2O3 in the infiltration agent were worse than those without La2O3, but the corrosion resistance was better than that without La2O3. Combined with the boronizing layer thickness, hardness, wear resistance, corrosion resistance and brittleness indexes, when 5% La2O3 is added to the boronizing agent, and the thickness of boronizing layer is moderate and the performance is optimal.