Hazard of Biodiesel (BDF) was studied. Biodiesel is a name for a variety of ester-based fuel made from vegetable oils. Recently importance of biodiesel is increasing, and its fires were sometimes reported. Therefore we studied on hazard of biodiesel comparing (petroleum) diesel oil and vegetable oil, raw materials of biodiesel. We found that biodiesel is auto-oxidized easily and ignites, and its flash point decreases when even small amount of methanol exists. And there are various raw materials to manufacture biodiesel, so we studied the difference of these materials, and their aging on safety.