The aromatic-compound additive in sulfuric acid (H2SO4) was found to promote isobutane/butene alkylation significantly. The selectivity of isooctane (C-8) and the alkylate research octane number increased by 23.46% and 5.09, respectively, when thiophene was added to H2SO4, especially. The proportion of trimethylpentanes can be improved from 58.57% to 84.58%, and the ratio of trimethylpentanes to dimethylhexanes was doubled, compared with that in the presence of H2SO4 alone. The improved performance in the presence of thiophene additive in H2SO4 is attributed to the improvement in carbenium-ion stability and the acceleration of hydrogen proton release, because of the existence of electron-rich pi-bond compounds, and the substitution of thiophene in the alpha site by the sulfonic group and isobutane/butene. Reaction parameters such as reaction temperature, reaction time, stirring speed, feed rate, and acid/hydrocarbon volumetric ratio were also investigated in detail. The synergetic mechanism of thiophene-H2SO4 on isobutane/butene alkylation was also speculated.