Effects of Ni addition on the mechanical properties of TiB2/SiC composites produced by RHP according to the chemical reaction Si + 2TiH(2) + B4C --> 2TiB(2) + SiC + 2H(2) up arrow have been investigated. The fracture strength and Vickers hardness reach their highest values at 2 wt % Ni content, but the fracture toughness reaches its lowest value at this Ni content. Suitable Ni content can enhance the boundary strength of the composites and it is supported by SEM observation and residual stress measurement by XRD technique. On the other hand, the results of the residual stress measurement show that the residual machining stresses in the surface layer of the specimens are not constant but change with different directions.