Previous studies on the transesterification of vegetable oil in supercritical methanol in a batch reactor resulted in a non-saponified product with high methyl esters content and high glycerol purity. For the continuous reactor, the high viscosity of vegetable oil might result in problems in the flow system. This study selected THF and hexane as co-solvents to reduce the viscosity of the vegetable oil. The effect of co-solvents was investigated in both 250 mL and 5.5 mL batch reactors by 2-replicate 2³ factorial design at temperatures from 290–350 °C, a molar ratio of methanol to vegetable oil from 12–42 and a molar ratio of co-solvent to vegetable oil from 0–5. The reaction time was fixed at 10 min. The products from the employed and unemployed co-solvent process were analyzed by GC-MS to confirm that the reaction among the vegetable oil, methanol and co-solvent was non-existent. However, some thermal cracking was observed in a 250 mL reactor at 350 °C and 30 min reaction time. The amount of co-solvents had no significant effect on methyl esters content and also did not allow the reaction to be completed under milder conditions. Thus, it was concluded that both THF and hexane were appropriate co-solvents to reduce the viscosity of vegetable oil for the continuous production of biodiesel in supercritical methanol.