In order to strengthen and toughen Si₃N₄ ceramics by addition of SiC-whiskers, we tried the control of interfacial properties between whisker and matrix and of orientation of whiskers. For interfacial control, the effects of Al₂O₃, ZrO₂ and carbon coating to the surface of whiskers were studied. The flexural strength and the fracture toughness of Si₃N₄ composite containing Al₂O₃-coated whiskers by decomposition of aluminum stearate, were stronger (1107MPa) and larger (10.2MPa·m^1/2) than those of the composite with noncoated whiskers. TEM observation revealed that Al₂O₃-coated whiskers in the composite had smoother surface than non-coated whiskers and that a film-layer was formed at the interface between whisker and matrix. For orientation control, the flexural strength and the creep rupture resistance at 1250°C of Si₃N₄ composite, reinforced with uni-directionally oriented whiskers by the doctor-blade method, were much stronger (1180MPa) and much larger than those of monolithic Si₃N₄ ceramics. After creep testing, the new stacking fault and dislocation in the whiskers were observed by TEM. On the basis of these results, the mechanisms of toughening and high-temperature strengthening of SiC(w)/Si₃N₄ have been discussed.