We established two good linear relationships between pK a and acidic atom’s electrostatic potential (MEP) in the gas phase by DFT methods at B3LYP/6-31+G(d,p) level of theory. We studied 13 experimentally well-characterized singly substituted benzoic acids to determine the best level of theory. A good linear relationship between MEP and the experimental pK a data has been revealed (standard deviation (STD)=0.081, R 2 =0.937 for model 1; STD=0.081, R 2 =0.934 for model 2), better than the computational results in the literatures. Our models also showed that it is reliable to estimate the pK a values of multiply substituted benzoic acids.