A better understanding of the roles of rare earth (RE) in heterogeneous catalysts is crucial for improving the properties of catalysts in petro-chemical industry. Fluid catalytic cracking (FCC) catalysts containing RE-incorporated Y zeolites with excellent stability and activity are continuously used to improve the conversion of heavy oil into transportation fuels. In this paper, the effect mechanism of Lanthanum (La) on stability and acidity of Y zeolites are elucidated in conjunction with their influences on catalytic performance. The improvement on stability of La-incorporated Y zeolites can be attributed to the La incorporation which bonds strongly with framework O-2 and O-3 atoms and strengthens the interaction between framework Al and its neighbouring O atoms. It is the enhanced stability restrains the release of framework Al and the formation of extra-framework Al, hence leads to a decrease of total L acid amount and an increase of total B acid amount, especially a clear increase in medium strength B acid amount (with P-31 NMR/TMPO chemical shifts at 62 and 58) and a significant decrease in strong B acid amount (with P-31 NMR/TMPO chemical shifts at 70 and 65) with the incorporation of La into Y zeolites. However, the total acid amount decreases significantly when La2O3 content is high. The increase of B acid amount and improved stability of LaHY zeolites, boost the conversion ability and isomerization ability of heavy oil, while resulting in poor olefins selectivity and coke selectivity caused by the enhancement of hydrogen transfer reactions. Therefore, Y zeolite containing appropriate La2O3 content should be considered to enhance heavy oil cracking ability while boosting light olefins production and optimizing coke selectivity. [GRAPHICS] .