Polynitrogen compounds (containing only nitrogen atoms) are promising candidates as High Energy Density Material(HEDM) for propellants and explosives. Polymeric nitrogen particularly for cubic-gauche nitrogen (cg-N) has the highest energy contents in polynitrogen compounds. From theoretical estimates, cg-N can provide a specific impulse of 500 sec with material density 3.9 g·cm^-3, which is a tenfold improvement in detonation pressure over HMX. cg-N was produced experimentally in 2004 by compression of molecular nitrogen in adiamond anvil up to a pressure of 140 GPa at a cell temperature higher than 2000 K, but it cann′t be restained at ambient condition If polymeric nitrogen can be obtained at ambient condition, this will be a milestone in the development of energetic materials. Therefore a brief review of cubic-gauche nitrogen (cg-N) and previous amorphous (semiconducting) polymeric nitrogen were overviewed in this paper. The properties of N₂/H₂ alloy, which was prepared with N₂/H₂ (volume ratio 2:1) mixture under 85 GPa at room temperature, were also introduced.