Densification of SiC-TiC composites without the aids of high pressure or sintering additives is very difficult owing to the covalent nature of SiC and presence of inert second particles (TiC). Therefore, sintering additives such as metals, oxides, and nonoxides are usually added to densify the composites [1–4]. The role of additives for the sintering of nonoxide ceramic composites can be regarded as not only densification aids but also key elements for the microstructural development, since the related properties of composites are influenced by the kinds and amounts of additives [5–7]. Several attempts have been published on in situtoughened SiC-TiC composites. Maupas et al. [8] investigated the fracture toughness of the SiC-TiC nanocomposites with needle-like microstructure that were fabricated via chemical vapor deposition; these researchers reported a maximum toughness value of 6.2 MPa ·m1/2 in SiC-15 mol% TiC nanocomposites. Chae et al. [9] fabricated SiC-30 wt% TiC composites by hot-pressing with the aid of 10 wt% Cr3C2; their specimens exhibited a maximum toughness value of 6.2 Mpa ·m1/2 in SiC-30 wt% TiC composites. Cho et al. [10] also fabricated in-situ toughened SiC-30 wt% TiC composites via a two-step process, hot-pressing with the aid of 7 wt% Al2O3 and 3 wt% Y2O3 at 1850 ◦C for 1 h and subsequently annealing at 1950 ◦C for 6 h; their microstructure consisted of uniformly distributed elongated α-SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase and their specimens exhibited a higher fracture toughness of 6.9 MPa ·m1/2 in SiC-30 wt% TiC composites. In this study, the effect of sintering-additive composition on fracture toughness of SiC-30 wt% TiC composites was investigated for five kinds of additive compositions; three kinds of Al2O3-Y2O3, an AlN-Y2O3 and an Y-Mg-Si-Al-O-N oxynitride glass. Commercially available α-SiC (A-1 grade, Showa Denko, Tokyo, Japan), β-SiC (Ultrafine grade, Ibiden Co., Ltd, Nagoya, Japan) and TiC (Grade C.A.S, H. C. Starck, Berlin, Germany) powders were used as starting powders. The mean particle sizes of the αSiC, β-SiC, and TiC powders were 0.45μm, 0.27μm, 1.40μm, respectively. A mixture of SiO2 (99.9% pure, High Purity Chemicals, Osaka, Japan), MgO (99.99% pure, High Purity Chemicals, Osaka, Japan), Al2O3 (AKP-30, Sumitomo Chemicals, Tokyo, Japan),