The acidity constants (Ka) of some 6-substitue-4-aminoquinazoline compounds were investigated theoretically. The gas and aqueous phase geometries and the possible tautomeric forms were defined with full geometry optimization by using B3LYP/6-31G(d) method for 6-substitue-4-aminoquinazoline derivatives. It was found that most stable forms are amino form and the first protonation occurs at ring nitrogen atom(N1).