To obtain CaO clinker with high density, low porosity, and good hydration resistance, the effect of the La₂O₃ content on physical properties, such as sintering property and hydration resistance of CaO clinker was researched in this paper. Apparent porosity and bulk density were measured by Archimedes principle with kerosene as the medium. Two methods were used to test hydration resistance of CaO clinker. One was carried out in a humidity cabinet at 20 ℃ with 90% of relative humidity. Another was the boiling method. SEM and EDS characterization analyses of CaO clinker were performed to observe the microstructural information and the hydration mechanism was put forward. The results show that: (1) the specimen with 0.79 mass% of La₂O₃ addition has lower apparent porosity, and higher bulk density; (2) the hydration mass gain of CaO clinker with 0.79 mass% La₂O₃ addition is lower, which is less than 1% after 20 days; (3) La₂O₃ mainly distributes in the grain boundary, and a small amount of La₂O₃ dissolves in grains. The average grain size of CaO specimens with 0.79 mass% La₂O₃ addition sintered at 1 450 ℃, 1 550 ℃, and 1 650 ℃, is 44.5 μm, 48.6 μm, and 63.5 μm, respectively; (4) solid solution is formed due to the similar ionic radius of La³⁺ and Ca²⁺, which promotes the sintering of CaO clinker and improves the hydration resistance simultaneously.