The extended adiabatic flame temperature method aims at predicting the lower flammability limits of fuel-air diluent :mixtures (including fuel mixtures and diluent mixtures) rionstoichiometric equation and nitrogen equivalent coefficients. A cubic function is introduced to describe the relation between the critical adiabatic flame temperature and inert volume-concentration. This method applies to ten compounds including methane, propane, iso-octane, ethylene, acetylene, benzene; methanol, dimethyl ether, methyl formate, and acetone for validation. A:good agreement is obtained between the predicted and measured values that the average relative deviations are all below 3.6%. The source's of error are mainly attributed to three causes: First, the adiabatic flame temperature method does not take into acount the heat losses from flame front to surroundings. Second, the test method atid determined criterion both have a significant impact on the experimental data. Third, the functional relationship between adiabatic flame temperature and inert concentration also influences the exactness.