Using the first principle calculations based on density function theory with the generalized gradient approximation and local density approximation，we studied electronic structure and mechanical stability of the recently synthesized nitrid e compound PtN with cubic structure. The results show that the PtN with zinc-ble nde structure is unstable. We further find that the rock-salt structure has a b igger cohesive energy than the zinc-blende structure, it is possible that a pha se transition from zinc-blende to rock-salt occurs under certain pressure.