Abstract: Pressureless sintered SiC (SSiC) is an important high temperature structure material, and the joining technique is one of the key techniques to extend its application. Joining of SSiC was carried out by the reaction joining process using polysiloxane (SR355) with active additive Ni nanopowder as joining materials. The pyrolysis of SR355 with Ni nanopowder was studied. The effects of the content of active additive and joining temperature on the joining strength of joints were investigated. The results show that the active additive promotes the pyrolysis of SR355. The maximum joining strength of the joints is obtained at the joining temperature of 1050℃ with the Ni nanopowder mass fraction of 1% and three times of the reinforcement treatments. The microstructure and composition of the interfacial area were analyzed by SEM and EDX. The results indicate that the interlayer is densified. The micrographs show good contact at the interfaces. The diffusion of  elements takes place at the interfaces. The active additive Ni nanopowder participates in the chemical reactions in the joining process， which contributes to the interfacial bonding，resulting in the increasing of joining strength of the joints.