A phase-changeable layer and a method of forming the same are disclosed. In the method, a first hydrogen gas is introduced into a reaction chamber into which a substrate is loaded at a first flow rate to form first plasma. A primary cyclic CVD process is carried out using precursors in the reaction chamber to form a lower phase-changeable layer having a first grain size on the substrate. A second hydrogen gas is introduced into the reaction chamber at a second flow rate less than the first flow rate to form second plasma. A secondary cyclic CVD process is carried out using the precursors in the reaction chamber to form an upper phase-changeable layer having a second grain size smaller than the first grain size on the substrate, thereby forming a phase-changeable layer. Thus, the phase-changeable layer may have strong adhesion strength with respect to a lower layer and good electrical characteristics.