Synthesis of biodiesel from soybean oil over solid base catalyst was investigated in this paper. In process of catalyst preparation, potassium fluoride was used as alkaline metal salt to load onto γ-Al2O3. Over this basic catalyst, transesterification reactions were carried out to prepare biodiesel using soybean oil and methanol as feedstock. According to experiments, the best reaction conditions were gotten as followed: load ratio was 72.68% (by weight), molar ratio was 12:1 (methanol/oil), reaction temperature was 338 K, mass of catalyst was 2% of oil (by weight), and reaction time was 3 h. Yield of biodiesel exceeds 99%. With X-ray diffraction (XRD) and DSC-TGA characterization, it was proved that new crystal phase synthesized by calcinations showed its favorable effect to the transesterification reaction.