This paper studies on the idea of decreation and poetic reality presented in the poetry of Wallace Stevens`. Denying God as an absolute truth, Stevens thinks human beings create an idea of God. His idea of God is the idea of supreme fiction which is a poetry, a work of imagination by a poet. Being created within the poet`s deepest imagination, the supreme fiction is taken as a poetic truth. This process from the created to uncreated is the process of "decreation." Decreation plays an important role in Steven`s poetic theory. Up to the present, only a handful of critics--J. Hillis Miller and B. J. Leggett-- commented on the issue. Even Miller and Leggett consider it in terms of abstraction that is quite different from Stevens` idea of decreation. This paper conforms to the notion of decreation as proclaimed by Stevens himself, and details Stevens` theory including the process of decreation. The paper employs textual analysis to explain the steps toward the making of supreme fiction as well as decreation. To sum it up, the steps toward supreme fiction are "abstraction" and "change." Abstraction is the act of mind, in which the poet accepts things as they are. And then, he/she transforms the abstracted reality through intensified imagination into a new reality. This new reality is a poetic reality and becomes a truth--a poetic truth through decreation. This poetic truth is not absolute, fixed, and final, but changeable, challengeable, and relative.