Charge distributions determining the electrostatic potential in monosubstituted benzenes are investigated. As a result, a new quantum chemical parameter for substituent effect, which would be useful in information coding (in terms of molecular electrostatic potential), is introduced. The curvature of charge distribution over the benzene clearly shows a good linear relationship with known experimental substituent constants, while the amount of charge transferred does not fully reflect the substituent effect. Because the curvature is a new and good quantum chemical parameter to describe the substituent effect, our results would be used as a strategy for design of new functional molecules such as molecular scale electronics using the electrostatic potential distributions.