Objective To study the efficiency of inhaling nitrogen monoxide in labor on pain relief and its effect on mother and fetus. Methods Two hundred term primiparas were randomly divided into the observational and control group.The control group was given the same treatments as that in the observational group except for inhaling of nitrogen monoxide. The degree of labor pains, duration of labor, mode of delivery, Apgar scores, the volume of intrapartum bleeding, the rate of postpartum hemorrhage and other outcomes were compared between the two groups. Results The scores of labor pains in the observational group was significantly lower(P