The relation between the speed of sound (u) in biodiesel and the change in Gibbs energy (G) has not been described in the literature. With the method of Gibbs energy additivity, the relation between u and G can be expressed as ln(u2) = G/RT + A, where R is the universal gas constant, T is the absolute temperature, and A is a constant. Further expansion of G into its enthalpy and entropy, and sub-dividing the molecule of a fatty and methyl ester (FAME) into groups of atoms, the final model is good for estimating the speed of sound in both FAME and biodiesel at various temperatures. Only the numbers of double bonds and carbon atoms of the fatty acid are required for the calculation.