A carbon-based solid acid catalyst was prepared by the sulfonation of carbonized vegetable oil asphalt This catalyst was employed to simultaneously catalyze esterification and transesterification to synthesis biodiesel when a waste vegetable oil with large amounts of free fatty acids (FFAs) was used as feedstock The physical and chemical properties of this catalyst were characterized by a variety of techniques The maximum conversion of triglyceride and FFA reached 80.5 wt.% and 94 8 wt.% after 4 5 h at 220 degrees C, when using a 16 8 M ratio of methanol to oil and 02 wt % of catalyst to oil. The high catalytic activity and stability of this catalyst was related to its high acid site density (-OH, Bronsted acid sites), hydrophobicity that prevented the hydration of -OH species, hydrophilic functional groups (-SO(3)H) that gave improved accessibility of methanol to the tnglyceride and FFAs. and large pores that provided more acid sites for the reactants (C) 2010 Elsevier Ltd. All rights reserved.