Objective To investigate the change of serum nitric oxide ( NO ) in premature rupture of membrane (PROM) and its clinical significance, so as to assess the role of NO in the mechanism of PROM . Method The serum NO in 40 advanced stage pregnant women,28 premature delivery PROM,40 unknown reason term pregnancy PROM ,40 term pregnancy ,40 term labor were detected and compared. Results The serum NO was higher in unknown reason term pregnancy PROM [(95.364±3.562) (u)ol/L] than that in term pregnancy [(75.020±3.416)(u)ol/L] and term labor [(31.457±3.859)(u)ol/L](P<0.01). The serum NO was higher in premature delivery PROM [(101.256±3.124)(u)ol/L] than that in advanced stage pregnant women [(80.363±5.112)(u)ol/L] (P<0.0l),but similar with unknown reason term pregnancy PROM (P = 0.086). Conclusions The serum NO in PROM of term pregnancy significantly is higher than that of advanced stage and term labor women,indicates PROM relating to the increase of NO level. Because the increase of NO links to infection,it suggests that PROM relate closely to the amniotic infection.