Theoretical study based on the Restricted Hartree-Fock calculations (RHF/3-21G) of the formation of Schiff base of para-methoxyaniline, para-methylaniline and anline with thiophene-2-carbaldehyde was conducted. The reaction mechanism wasfound to involve two steps, namely: (1) formation of a carbinolamine and (2) dehydration of the carbinolamine to give the finalschiff base, through six-membered ring transition states. The carbinolamine is intermediate and dehydration is rate determiningstep of the reaction. The formation of Schiff base between the para-substituented aniline and thiopheneadehyde requires thecontribution of one auxiliary water molecule as a true reactive in order to facilitate proton transfer, and allows the nucleophilicattack of the incoming amine to carbonyl group. The para-methoxy substituent has less energy barrier than the other twosubstituents, according to more electron withdrawing effect.