A general theory is developed which enables closed-cup flash points of mixtures of flammable and non-flammable liquid to be estimated from a knowledge of a certain properties of flammability diagram by thermodynamic method. The estimated equations is shown the effect of both the flame inhibiting properties of the vapor of the non-flammable component and the relative volatility of that component. The vapor phase flame inhibition effect results in a even greater elevation of flash points than the relative volatility of that component. Especially in cases of similar vapor phase flame inhibition of the nonflammable component, the relative volatility is affected greater elevation of flash points(extinguishing effect).