Sulfated zirconia is able to effectively catalyze the reaction between aniline and trimethyl orthoformate to produce formamidine with a product yield of 97% at 313 K. N-phenylformimidate is proposed as the reaction intermediate, which reacts further with aniline to produce formamidine. Less degree of acid sites poisoning by the reaction mixture due to the weaker acidity of sulfated zirconia as well as its mesoporosity seem to play important role in this catalyzed reaction.