The geometries, electronic structures and related properties of SimN8-m(0 < m < 8) clusters are studied using density functional theory (DFT) with hybrid functional B3LYP. The calculated results reveal several trends. For any stoichiometric clusters, the lowest energy isomers with an alteration of N and Si atoms are favourable in energy if the numbers of Si and N atoms are large enough to form ... Si-N-Si-N... alteative chains. The bond lengths of single Si-N bonds are very close to the corresponding values of the bulk and other Si-N clusters. The geometries for N-rich and Si4N4 clusters are planar structures, but three-dimensional structures are favourable in energy for Si-rich clusters. With the increase of m, the isotropic polarizability and average polarizability increase, the total binding energies generally decrease, the HOMO-LUMO gap and vertical ionization potential oscillate with increasing number of valence electrons,and their values with even valence electrons are larger than those with odd valence electrons. The atomic charges, IR and Raman properties are also reported.