To shed more insight into the regioselectivities observed in the reactions between 3,5-diaminopyrazoles and N-acyl imidates or N-ethoxy imidate yielding the corresponding substituted pyrazolo[1,5-a]-[1,3,5] triazines or pyrazolo[1,5-a][1,3,5] triazinones, DFT calculations are carried out using B3LYP/6-31 + + G (d, p) method. The favored mechanism and the experimental regioselectivity of this reaction are rationalized by calculations of activation energy, natural atomic charge, and Fukui indexes derived from density functional theory. The present study shows that the experimental trends of the relative reactivities and regioselectivities of these reactions are correctly predicted using these computational levels.