The geometries and energies of SinC and SinC- (n=2-5) were investigated with ab initio MO calculations including electron correlation. The Moller-Plesset perturbation theory was used for n=3-5. The configuration interaction (CI) calculations were carried out for n=2 and 3. The most stable isomers of the neutral SinC clusters have the structure in which an Si atom having the largest sp hybridization in the corresponding Si-n+1 cluster is replaced by a C atom. For their anions, the structures in which an excess electron is distributed avoiding a C atom become stable. Moreover, the adiabatic electron affinities of SinC and vertical detachment energies of anionic SinC- were computed. They reproduced the trends of photoelectron spectra of SinC- reported previously.