Most studies focused on Marx's philosophical thought,laid much emphasis on its source from German classical philosophy,and consequently neglected other connections between Marx's thought and other philosophies,especially the one of French enlightenment thought.This paper seeks for the connection between Marx's thought and the enlightenment thought through comparing Marx's thought about alienation with Roussear's alienation thought,so as to make effort to understand Marx's thought comprehensively.It is discovered that Marx had accepted the concept of alienation from Roussear,developed it and used it as his weapon to analyze and criticize the capitalist society,which in turn strengthened the connection between Marx's thought and the enlightenment thought.