For the self-reinforced HP Si
3N
4 ceramics, the effects of(Y
2O
3+La
2O
3) wt% and the ratio between Y
2O
3 and La
2O
3 on the mechanical properties and microstructure were studied. Analysis of toughening processes indicated that both crack deflection and -Si
3N
4 bridging mechanisms can contribute to the fracture toughness of HP-Si
3N
4 ceramics. Inaddition, the oxidizing behavior and feature of subjecting to thermal cycling were studied in this paper.