We have examined the effects of two different solvents on the reaction rates in acetylation of aniline. Diethylether and acetic acid were used as solvents, and the reaction rates using a microreactor were compared to those with the batch method. The reaction rate constants with the acetic acid solvent were larger than those with the diethylether solvent in both the microreactor and the batch method. Moreover, in acetic acid, we reduced the reaction time to about 1/14 using the microreactor. The reaction rates indicate that this reaction was nearly a reaction-controlled process with diethylether used as the solvent, while the reaction process was diffusion-controlled when acetic acid was used as the solvent. We have also performed quantum chemical calculations including solvent effects to understand the mechanism of the solvent effects on reaction rates in solutions. Acetic acid with the larger acceptor property gives more stability to the acetic acid ion species in the transition states. The calculated reaction rate was higher when using acetic acid as the solvent because the activation energy was smaller than that obtained with diethylether. The calculated dependence on the solvent effects is qualitatively in agreement with the experimental value with the use of a microreactor. Therefore, it can be concluded that changing the solvent used is an effective way to accelerate reaction rates.