In this work the correlation between hardness and basicity (pKa) of substituted anilines and N-Alkylated anilines has been studied. The hardness values of PhNH2,o-, m-, and p- substituted, NH2-PhNH2,Me-PhNH2, Cl-PhNH2, NO2-PhNH2 and N-alkylated anilines (PhNHMe, PhNHEt, PhNHPh) have been calculated using KoopmansÂ relation. The calculation of the energies of HOMO & LUMO of the above compounds has been done semi-empirical quantummechanically (usingAM1&PM3 semiempiricalHamiltonian) with the help ofArgusLab4.0 software. It is found that hardness value of aniline decreases along with substitution. The high correlation between pKa values and hardness of substituted anilines and N-Alkylated anilines indicate the hardness bears direct relationship to the basicity of substituted anilines.