In this study, the production of biodiesel from soybean oil by transesterification was carried out over sodium aluminate as a heterogeneous catalyst. The solid base showed high catalytic activity for methanolysis reaching a 93.9% yield under optimal reaction conditions (reflux temperature, 1.5 wt% of catalyst, 12:1 molar ratio of methanol/oil, and 50 min). The catalyst treated at different temperatures was characterized by inductively coupled plasma-optical emission spectroscopy (ICP-OES), X-ray diffraction (XRD), thermogravimetry/differential thermogravimetry (TG/DTG), and Fourier transform infrared spectroscopy (FT-IR). The reaction contained homogeneous and heterogeneous contributions at the same time. Removing water and carbon dioxide was an effective way to elevate the catalyst stability in methanol.