In this study, catalytic transesterification of Parkia seed oil with methanol to biodiesel was studied. Attapulgite clay, treated with HCl and then functionalized with C4H6O4KNa and Na2C2O4 was used as a solid base catalyst. Having screened the catalytic action of attapulgite treated with HCl and functionalized with C4H6O4KNa at different concentrations, 4 M solution C4H6O4KNa was chosen for the functionalization and the results were compared with data obtained with 4 M Na2C2O4. Impregnation method was applied to prepare catalysts. Functionalized catalysts dried and calcined at 450°C for 4 h were characterized by Hammett indicators, nitrogen adsorption isotherms, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and thermogravimetric analyses. The performance of HCl treated attapulgite and functionalized with 4 M C4H6O4KNa (4NK/HCl-Atta-4) was superior both to HCl treated attapulgite functionalized with Na2C2O4 (4NC/HCl-Atta-4) and to HCl treated attapulgite (HCl-Atta-4, reference). Maximum yields of biodiesels of 94.7 and 92.2% were obtained by using 4NK/HCl-Atta-4 and 4NC/HCl-Atta-4, respectively. The optimum reaction conditions were: ratio methanol : oil = 9 : 1; catalyst amount, 6 wt %; and reaction temperature, 60°C; reaction time, 6 h. The catalysts preserved catalytic activity after 5 runs of reuse. The properties of the biodiesels were comparable with those required by American and European Union standards and they could be considered as a realistic alternative fuel to diesel.