A peanut shell-derived solid acid of amorphous carbon bearing SO3H, COOH, and phenolic OH groups has been prepared by a one-step process. The effects of preparation conditions on the activity of the solid acid were investigated. The catalysts were characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis. This catalyst showed high activity in the esterification of oleic acid with methanol. Under the optimal reaction conditions, a biodiesel yield of 98% could be obtained. Additionally, after six repeated uses, more than 95% yield of biodiesel was obtained, which indicated that the catalyst has a highly operational stability.