This invention discloses a method for making rice bran flour by treating rice bran under high pressure and high temperature for a long period of time in two separate steps, the method comprising: husking and grinding the rice, then collecting the rice bran; inactivating the lipase of the rice bran; treating the rice bran under high pressure and high temperature; drying and screening the rice bran; treating the rice bran flour so obtatined under high pressure and high temperature again; and cooling the rice bran. Said rice bran flour made from said method is effective in lowering glycosylated hemoglobin and increasing insulin of diabetes mellitus patients.