Mendelssohn, who was a representative philosopher of Germany in the 18th century, is now neglected merely as a dogmatist philosopher who was criticized by Kant or as a popular philosopher who tried to propagate rationalist philosophy. However, in order to understand the 18th-century German philosophy, Mendelssohn’s philosophy needs to be studied and evaluated more thoroughly in itself. Above all, the study of Mendelssohn’s philosophy can make the 18th-century German philosophy map much more abundant, which has been somewhat simply grasped according to the dichotomous view with regard to the authority of reason. Mendelssohn elaborated his unique philosophy according to which, on the one hand, he as a rationalistic philosopher defended the authority of reason, and on the other hand, he as a jewish philosopher demanded that reason be tolerant for the traditional belief. In order to confirm this, we will first analyze Mendelssohn's position on the Enlightenment in comparison with Kant's. Furthermore, we will examine Mendelssohn's Jerusalem and show that his unique position on the Enlightenment does not originate from a popularist view, but from an existential view based on his Jewish faith. And finally, we will evaluate Mendelssohn's Jewish Enlightenment and present a future task for the study of Mendelssohn' s philosophy.