The flash point (FP) of binary mixtures formed by fatty acid methyl esters (FAMEs), which are the major compounds of methyl biodiesel, is investigated in this study, and the nonideality of the liquid phase was calculated with the ideal model, UNIFAC, UNIFAC-Dortmund (UNIFAC-Do), and NIST-UNIFAC models. The different models used to calculate the activity coefficient were compared through the FP result that was estimated using Liaw's approach. The experimental results show that the larger the carbon chain of FAME is, or the richer the solution in the heavier compound is, the higher the FP of the compound and solution is. The results also show that the ideal model is the best one between those tested in this study because the solutions studied here can be considered ideal, and the calculated FP temperature of the binary mixtures presented an average root-mean-square deviation equal to 0.67% using this model. It was observed as an abrupt decrease in the FP value of the mixture with small addictions of the light compound in the region that is rich in the heavier compound, remarkably when the difference in the length of the alkyl carbon chain of these components is large. This behavior is an important factor that must be considered in safety and transportation cases.