Ab initio calculation is performed to estimate the substituent effects for Para-substituted pyridines. Electrostatic potentials are obtained from ab initio molecular orbital wavefunctions of optimized structures for substituted pyridines. Electrostatic potentials are computed to be minimum at nitrogen atom of pyridines. The potential minima are good correlated with the substituent constants, ${sigma}_p$ and with the ${Delta}pKa$. It is shown that the electrostatic potential minima can be used as a measure of substituent effects.