It is self-directed learning(SDL)that has been defined or used variously by many researchers or practitioners in adult education until now. There might be no doubt that autonomy is recognized as the principle in adult education without quoting the theory of andoragogy by Lindeman, E. D. On this premises, such researchers have been trying to describe the characteristic of adult education using the theory of SDL. Properly speaking, I had better read through all the works which they wrote in the past and present a bird's eyes view. But the aim of this essay is to focus faithfully on the theory of SDL defined by P. C. Candy, which is based on Constructivism.