The SiC/Al₄SiC₄ composites with the improved mechanical properties and thermal conductivity were fabricated by the in-situ reaction of polycarbosilane (PCS) and Al powders using spark plasma sintering. The addition of 5 wt% yttrium (Y) sintering additive was useful to obtain fully dense samples after sintering at a relatively low temperature of 1650 °C, due to the formation of a liquid phase during sintering. The average particle size of the in-situ formed SiC was ~300 nm. The fracture toughness (4.9 MP·m^1/2), Vickers hardness (16.3 GPa), and thermal conductivity (15.8 W/(m·K)) of the SiC/Al₄SiC₄ composite sintered at 1650 °C were significantly higher than the hardness (13.2 GPa), fracture toughness (2.16 MPa·m^1/2), and thermal conductivity (7.8 W/(m·K)) of the monolithic Al₄SiC₄ ceramics. The improved mechanical and thermal properties of the composites were attributed to the high density, fine grain size, as well as the optimized grain boundary structure of the SiC/Al₄SiC₄ composites.