Phospholipase D (PLD) and its product phosphatidic acid (PA) function as pleiotropic factors in the regulation of cancer progression, which includes the promotion of cell growth, survival, cell migration, and angiogenesis. Chemical inhibition of the enzymatic activity of PLD1 and PLD2, or genetic down-regulation of their expression both induce tumor suppression, while the compounds for PLD expression inhibition are barely reported. Here, we astonishingly found that the anti-hyperglycemic agents biguanides decreased PLD1 protein expression in cancer cells and further investigated its underlying regulatory mechanism.