The aim of this work is the determination of protonation site of hydrazobenzene derivatives in gas phase by using ab initio calculations. Optimizations of neutrals and N-ptotonated species are effectuated at the DFT/B3LYP method with the 6-311+G(d,p) basis set. Introduction of natural bond orbital NBO population analysis, fukui functions and local softness s( f )k are calculated to determine the most site of hydrogen attack. Energetic comparison between N1 and N2 protonated compounds were effectuated to determine the most stable protonated species. The protonation occurs preferentially on the nitrogen atom of the substituted cycle. Structural analysis of all the neutrals and N-protonated species correspondents is effectuated to analyze the geometry changes between the N-protonated and neutral ones. Different global chemical parameters of reactivity have been calculated to determine a reactive compound.