Gestational diabetes mellitus (GDM) is considered as the early stage of type 2 diabetes mellitus. In this study, we compared the demographic and clinical data between six GDM and six NGT (healthy controls) and found the HOMA-IR was increased in GDM. Previously, many researches had proved that omental adipose tissues dysfunction could induce insulin resistance. Thus, in order to investigate the cause of the insulin resistance in GDM, label-free proteomics was used to discover differentially expressed proteins in omental adipose tissues between GDM and NGT. A total of 3529 proteins identified, including 66 significantly changed proteins. Adipocyte plasma membrane associated protein (APMAP also called C20orf3) was one of changed proteins and down regulated in GDM omental adipose tissues. Further, mature 3T3-L1 adipocytes were used to simulate omental adipocytes and we found that inhibited the expression of APMAP by RNAi would impaired the insulin signaling and activated the NFkB signaling in adipocytes. These results indicated that the decreased of APMAP in mental adipose tissues may be important in process of the insulin resistance in GDM.