Abstract: This paper shows some examples of gas rich in nitrogen in the marine strata of South China, explains their distribution characteristics, and discusses various source and formation mechanism of nitrogen. It is pointed out that, nitrogen in natural gas usually comes from biological detritus decomposed by bacteria, low temperature pyrolysis of kerogen, gas migration, nitrate-mineral decomposition and atmosphere, but nitrogen in marine strata is mainly from atmosphere. Nitrogen in natural gas indicates connection of surface with underground, therefore, it could be an effective index for oil-gas reservoir.