A series of solid bases (KF/Mg-La) with different Mg/La mass ratios and varied KF loadings were prepared, and applied to biodiesel synthesis by transesterification of cottonseed oil. ICP, XRD, CO2-TPD, TPDE, and FTIR were employed to characterize the catalysts in order to reveal the correlation of the composition with the structure and the basicity of the solid bases. The catalyst with Mg/La mass ratio of 3% and 30.9% (wt/wt precursor) KF loading possessed the highest catalytic activity. Under the optimal reaction condition of methanol/oil molar ratio of 12:1, reaction for 1 h at 65 degrees C, and 5% (wt/wt oil) catalyst amount, the TOF on 30.9% KF/M3L could reach 3.28 x 10(-2) mol s(-1) mol(-1). The Arrhenius activation energy was evaluated to be 54.74 kJ.mol(-1).