We present a theoretical investigation on the structural evolution and electronic properties of Nb2Sin-/0 (n = 13-20) clusters. The two Nb atoms tend to stay close and occupy the high coordination sites. Their most stable structures can be described as a central axis of Nb-Nb bond surrounded by the Si n frameworks. The structural evolution between Nb2Sin- anions and Nb2Sin neutrals is markedly different. Nb2Si19- is found to be the critical size of forming Nb-2-endohedral structure for anionic clusters, whereas the Nb-2-endohedral structure forms at n = 20 for neutral clusters.