Nephropathy is one of the most common and severe complications of diabetes mellitus. The mechanism of diabetic nephropathy, however, remains incompletely understood. To elucidate the mechanism of diabetic nephropathy, we focus on the role of a vitamin K-dependent growth factor, growth arrest-specific gene 6 (Gas6), and its receptor Axl in the pathogenesis of diabetic nephropathy. We used streptozotocin (STZ)-induced diabetic rats and mice as a model of diabetic nephropathy and examined the role of Gas6 and Axl in the development of diabetic nephropathy. We also studied signaling mechanisms involved in mesangial hypertrophy characteristic of the early phase of diabetic nephropathy in vitro. After 12 weeks of STZ injection, the glomerular expression of Gas6 and Axl was increased along with the phosphorylation of Akt, p70 S6 kinase, and 4E-BP-1. Administration of warfarin, which inactivates Gas6, inhibited mesangial and glomerular hypertrophy and the increase in albuminuria in STZ-rats. Warfarin treatment also inhibited the phosphorylation of Akt, p70 S6 kinase, and 4E-BP-1. To demonstrate the specific role of Gas6, we showed that these findings were recapitulated in STZ-induced Gas6-knockout mice and confirmed the role of Gas6 in the development of diabetic nephropathy in vivo. In vitro stimulation of mesangial cells with Gas6 resulted in mesangial cell hypertrophy. Stimulation of the cells with 25 mmol/l of glucose increased the expression of Gas6/Axl and mesangial cell size compared with that with 5.6 mmol/l of glucose. LY294002 and rapamycin blocked Gas6-induced activation of the Akt/mTOR pathway and mesangial hypertrophy. Thus, we have found a novel mechanism of glomerular hypertrophy through the Gas6/Axl-mediated pathway in the development of diabetic nephropathy, where the Akt/mTOR pathway is a key signaling cascade in Gas6-mediated mesangial and glomerular hypertrophy. Inhibition of the Gas6/Axl pathway in diabetic patients might be beneficial to slow down the progression of diabetic nephropathy.