Objective We recently reported that accumulation of the transcription factor C/EBPβ in β cells causes endoplasmic reticulum (ER) stress and reduced β-cell mass, leading to β cell failure and established β cell-specific C/EBPβ transgenic (TG) mice (J Clin Invest, 2010). These TG mice showed a non-fasting glucose level of approximately 200 mg/dL with reduced β-cell mass. Therefore, we considered TG mice indicative of a model of mild diabetes. Next, we showed that the DPP-4 inhibitor vildagliptin (Vilda) can preserve pancreatic β-cell mass in TG mice as it reduces C/EBPβ levels, reduces ER stress, and ameliorates insulin signaling (J Mol Endocrinol, 2012). We subsequently investigated the mechanism of modulation of C/EBPβ expression. Methods In vitro, MIN6 cells were treated with tunicamycin, exendin-4, metformin and AICAR. In vivo, TG and wild-type (WT) mice were orally treated with Vilda, with or without metformin (Met) in conjunction with vilda. Results Incubation of MIN6 cells with exendin-4 or metformin resulted in the activation of adenosine monophosphate-activated protein kinase (AMPK), which suppressed C/EBPβ expression. Furthermore, co-expression of dominant-negative AMPK increased the expression of C/EBPβ. In vivo, treatment with Vilda markedly ameliorated hyperglycemia in TG mice in association with the increase of AMPK activity in pancreatic β cells. In addition, the Vilda + Met-TG mice exhibited an increase in pancreatic β-cell mass when compared to the Vilda-TG mice, concomitant with reduced expression of C/EBPβ and further increase in AMPK activity. Conclusion The modulation of C/EBPβ expression by AMPK could regulate pancreatic β-cell mass.