The stability of water-in-oil emulsion in oil based drilling fluid was investigated by low field NMR(low field nuclear magnetic resonance).Firstly,the transverse relaxation time T2 of white oil and pure water were obtained by the CMPG pulse sequence,then,a further qualitative analysis on the T2 distribution curve of water in oil emulsion was conducted by introducing the relaxation reagent MnCl2?4H2O,the relaxation peak located between 10 ms and 1 000 ms corresponded to the overlapping peaks of moderate movable free water and white oil,which was defined as emulsion relaxation peak.The relaxation peak located between 1 000 ms and 10 000 ms corresponded to that of the high degree movable free water.Thus,in this paper,the relaxation peak area ratio and relaxation peak spacing-were selected as quantitative indicators to investigate the influences of relaxation reagent,oil to water ratio and aging time on the transverse relaxation time T2 distribution curve of emulsion.In addition,low field NMR was also used to test the relative oil content in oil based drilling fluids emulsion.Experimental results showed that the low field NMR is an efficient,fast and accurate analysis technique to reflect the stability of oil-based drilling fluids emulsion,and it also could be used to test the relative oil content in oil-based drilling fluids emulsion or crude oil efficiently and accurately.