Based on the principle of detonation and chemical thermodynamics, the lower explosive limits (L_EL) of organic compounds were studied, and two methods were developed to predict the lower explosive limits of organic compounds by using the data of combustion heat or the composition of the compounds. The calculated results showed that the predicted L_EL agreed with the experiment data satisfactorily, and the mean absolute deviations were 0.161 and 0.210. Not only can the methods be used to predict the L_EL of organic compounds, but can help to understand the quantitative relation between L_EL and the heat of combustion or composition.