In the present paper, the effect of La2O3-Y2O3-Al2O3 (LYA) and La2O3-Y2O3-MgO (LYM) additives on the mechanical properties of hot pressed SiC whisker-reinforced Si3N4 matrix composites and the relationship between whisker content and additive content were investigated in detail. The results show obvious strengthening in the case of LYA additives containing ≤2.4 wt% Al2O3. When the Al2O3 content of the additive was > 1.2 wt%, the fracture toughness decreased accompanied by decreased interfacial debonding and whisker pull-out. With LYM additives containing 2.4 wt% MgO, both the room temperature strength and toughness increased. A combination of Y2O3 + MgO (YM) with LYA additives produced substantial improvements in strength and toughness, up to 1050 MPa and ∼11 MPa√m, respectively. The Weibull modulus of a SiC whisker-reinforced composite was found to be greater than that of either the unreinforced Si3N4 or Si3N4 containing SiC particles. The experiments also demonstrated that in order to achieve full density the additive content had to be increased with increased whisker content. Preliminary but encouraging results on strength retention at elevated temperatures were also reported.