Glycerolysis can be a useful alternative for lowering free fatty acid (FFA) content present in waste cooking oil. In the present work, the effect of mass ratio of co-solvent hexane to oil on glycerolysis of waste cooking oil was investigated to enhance the miscibility of the oil and glycerol phases. The experimental results showed that the addition of hexane as co-solvent affected the glycerolysis reaction rate. However, a suitable amount of co-solvent must be added to achieve an optimum of FFA conversion. The use of 0.125:1 of mass ratio of co-solvent to oil leads to higher rate constant of approximately two times compared to glycerolysis without a co-solvent. The glycerolysis reduces the FFA content, peroxide and saponification values of oil; however, it does not change the density and kinematic viscosity as well as the fatty acid composition of oil.