Viscosity is one of the most significant properties to affect the utilization of biodiesel fuels. This paper presents a method, which has been verified experimentally, for predicting the viscosities of biodiesel fuels from the knowledge of their fatty acid composition. The applicability of a logarithmic mixture equation was verified using controlled mixtures of standard fatty acid esters and natural biodiesels. Several binary, ternary and quaternary mixtures of fatty acid ethyl ester (FAEE) gas chromatography (GC) standards were formulated. Their viscosities were predicted from their component values and were within +/- 3.7% of their measured values. The fatty acid compositions of six typical oils were simulated by mixing fatty acid methyl ester (FAME) standards in appropriate amounts. Viscosities of these mixtures were also predicted within +/- 2.1% of their measured values. Five biodiesel types were produced from natural oils and the logarithmic equation was applied to predict their viscosities. An average prediction error of +/- 3% was obtained for these samples. The viscosities of fifteen biodiesel types were then predicted based on their fatty acid composition as published in the literature and were found to vary as much as 100% This is most likely a principal contributing factor to the variation in performance of some biodiesel fuel types. The viscosity of biodiesel fuels reduce considerably with increase in unsaturation, Contamination with small amounts of glycerides significantly affects the viscosity of biodiesel fuels.