Several heterogeneous catalysts have been utilized for biodiesel synthesis from low-grade oils containing free fatty acids (FFA) in a two-stage esterification and transesterification method in previous studies. In this study, NaOH-bentonite impregnated with graphene oxide (GO), as a novel bi-functional heterogeneous catalyst for simultaneous esterification and transesterification was developed and characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, field emission scanning electron microscopy (FESEM), N-2 adsorption measurement to obtain the surface area (BET) and temperature programmed desorption (TPD NH3 and CO2). The effect of reaction time (h), reaction temperature (degrees C), methanol-to-oil ratio and catalyst amount (wt%) was studied. All experiments were performed at an optimized GO-to-NaOH-bentonite ratio of 1:20. Impressively, the GO-NaOH-bentonite catalyst exhibited excellent catalytic performance and resulted in 89.5 wt% conversion of free fatty acids and 98.5 wt% overall biodiesel yield after 4.5 h reaction time with catalyst amount of 6 wt%, reaction temperature of 62 degrees C and 6:1 methanol-to-oil ratio. Results showed that the presence of Na2O enhanced the low-strength basic sites of the catalyst at 150-200 degrees C, which played the key role in transesterification of rapeseed oil. It was also found that, the presence of GO on NaOH-bentonite, increased the overall acid strength of the composite catalyst by providing more Bronsted acidic sites. The acidic-basic nature of GO-NaOH-bentonite improved the accessibility of methanol to oil and FFA with the increased pore size of the composite catalyst.