A series of surface-modified SiO2 nanoparticles (NPs) were designed and prepared by combining a guanidine group (1,1,3,3-tetramethylguanidine [TMG]) as the base catalytic functionality and n-alkyl chains as the hydrophobic functionalities. These particles comprising either n-butyl, n-octyl, n-dodecyl, or n-hexadecyl chains (C-n-SiO2-TMG, n = 4, 8, 12, 16), can stabilize soybean oil-in-methanol Pickering emulsions, and be used as interfacial catalysts in the transesterification reaction for biodiesel production. It is shown that transesterification is possible in all emulsions stabilized by the C-8-SiO2-TMG catalyst. For the C-8-SiO2-TMG catalyst, the highest conversion of 66.7% is obtained at a catalyst concentration of 7 wt % after the reaction time of 5 h at 70 degrees C. The apparent activation energy (E'(a)) is estimated to be 26.8 kJ.mol(-1), leading to a high reaction rate. Interestingly, the most efficient catalyst is C-12-SiO2-TMG NPs with the balanced amphiphilicity and chain length, which can both improve the interfacial area and lower the transfer limitation.