Surface tension (gamma) is an important physical property of biodiesel for atomization and complete combustion in the diesel engine. In this study, the Martin's rule of free energy additivity is extended to cover the free energy of interfacial interaction of a liquid. Finally, an equation (gamma = 60.211 - 0.4307z - 0.1125T + 0.00207zT + 3.676m - 0.00893mT) is derived for estimation of the surface tension of an individual fatty acid methyl ester (FAME), where T, z and m are absolute temperature, number of carbon atoms and number of double bond(s) of fatty acid, respectively. When the proposed equation was used for estimation of the surface tension of FAMEs, the deviation range was 0.03-5.54% and the average absolute deviation (AAD) was 1.32% at temperature between 20 and 100 degrees C. Also, the proposed equation can be used for estimation of the surface tension of a biodiesel. In this case, the average carbon numbers (z(avg)) and average number of double bonds (m(avg)) must be used in place of z and m, respectively. Thus, the prior knowledge of surface tension of individual FAME is not necessary. The estimated surface tensions of biodiesels at temperatures between 30 and 80 degrees C were in good agreement with the literature values. The deviation range was 0.10-5.70% and the AAD was 2.09%. (C) 2014 Elsevier Ltd. All rights reserved.