A novel magnetic carbonaceous acid catalyst was synthesized from glucose and iron chloride by a new method of double hydrothermal precipitation at 180 degrees C and pyrolysis at 400-800 degrees C, and subsequent sulfonation at 150 degrees C. Its crystalline phases, magnetic saturation (Ms), morphology, specific surface area, pore volume, functional groups, thermal stability, elemental composition and total acid density were analyzed with various techniques. It was found that catalyst carbonized at 600 degrees C (AC-600-SO3H@Fe/C) had high acid density (2.79 mmol/g) and strong magnetism (Ms: 14.4 Am-2/kg) for direct production of biodiesel from Jatropha oil with high acid value (17.2 mg KOH/g) by single-factor optimization. With the catalyst, 90.5% biodiesel yield was achieved at 200 degrees C with 3 cycles (>90% yield) and 96.3% catalyst recovery rate. The magnetic catalyst directly esterified and transesterified high acid value oil without pretreatment with high biodiesel yield and easily separated for three recycles with little deactivation. It could also find other applications such as pretreatment of oils with high AV and hydrolysis of biomass. (C) 2015 Elsevier Ltd. All rights reserved.