Objective To investigate the relationship between the expression of glucose regulated protein 78 (GRP78) in the placental trophoblast cells and the pathogenesis of gestational diabetes mellitus (GDM).Methods All the patients were recruited from Qingdao Municipal Hospital from May 2013 to May 2014.Among them, fifty women with GDM were assigned to the GDM group, and fifty healthy women were defined as the control group.All of them received cesarean section because of breech presentation, contracted pelvis, scarred uterus or on mother's demand.Real-time PCR was conducted to analyze the expression of GRP78 mRNA in the trophoblasts.Immunohistochemistry was performed to detect the localization of GRP78 protein in the placentasl trophoblast cells.Results (1) GRP78 mRNA expressed in the cytoplasm of trophoblasts of both the GDM group and the control group.The GRP78 mRNA levels in the GDM group and the control group were 15.6±0.4 and 6.0±0.7, respectively.The relative expression level of GRP78 mRNA in the GDM group was 2.6 times of that in the control group, with statistically significant difference (P＜0.01).(2) The expression of GRP78 protein was found in the cytoplasm of the trophoblasts of the GDM group.It showed in deep, light brown or yellow after staining, according to the expression degree.The expression of GRP78 protein was also found in the cytoplasm of the trophoblasts of the control group, but it mainly showed yellow color (38/50).The strong positive rate of GRP78 protein in the GDM group (96％, 48/50) was higher than that in the control group (22％, 11/50;P＜0.01).Conclusion The expression of GRP78 increased in the placental trophoblast cells of GDM patients.It might suggest that GRP78 had some effect on the pathogenesis of GDM.