Forty-four pregnant female SD rats（aged 8 weeks） were randomly divided into normal control group and GDM model group. On the first day after pregnancy， the rats in GDM group were intraperitoneally injected with 35 mg·kg^－1 streptozotocin（STZ） to establish the GDM rat models. The rats in normal control group were intraperitoneally injected with the equal volume of citric acid-sodium citrate buffer. After delivery， the diaphragm contractile force of the newborn rats in two groups was measured in vitro. Meanwhile，the cross section area of the diaphragm fiber was detected by immunofluorescence method. Routine pathological staining and transmission electron microscope （TEM） were used to observe the phathomorphology and ultrastructure of diaphragm tissue of the newborn offsprings of the rats. The levels of malondialdehyde（MDA） and carbonylated protein and the activities of superoxidase dismutase（SOD） and catalase（CAT） in the diaphragm tissue were measured with commercial kits.

The results of contractile force detection in vitro showed that compared with normal control group， the diaphragmatic maximum tetanic force， maximum tension force， and contractile force under different stimulation frequencies of the newborn offsprings in GDM group were all significantly decreased（P<0.05）， and the diaphragmatic fatigue index was increased （P<0.05）. The immunofluorescence detection results showed that the cross section areas of type Ⅰ and type Ⅱ fibers of the newborn offspring rats in GDM group were significantly smaller than those in normal control group （P<0.05）. The HE staining results showed the neatly and densely arranged diaphragm fibers， intact nuclear without obvious cellular swelling， and clear nucleus staining in diaphragm tissue of the newborn offspvings in two groups. The TEM observation results showed that the diaphragm fibers in normal control group were neatly arranged with regular and visible light-dark bands or “Z” lines， and the mitochondrial structure was intact with partitioned and visible ridges； however， the diaphragm fibers in GDM group were arranged with irregularly or even the light-dark bands and “Z” lines were disappeared，the amount of mitochondria was decreased，and the signs of mitochondrial apoptosis or vacuolation were found in diaphragm tissue of the newborn offsprings GDM group. Compared with normal control group， the levels of MDA and carbonylated protein in diaphragm tissue of the newborn offspring rats in GDM group were significantly increased（P<0.05）， and the activities of SOD and CAT in GDM group were significantly decreased（P<0.05）.

GDM can cause the decrease of diaphragmatic contractile force， reduction of cross section area of diaphragmatic fibers and changes of diaphragmatic ultrastructures，and the increase in oxidative stress level is one of the reasons that affect its diaphragm function.