When natural rubber (NR) with low cross-linking density is quickly stretched to high draw ratio, occasionally, stable strain-induced crystals form, resulting in a “shape memory” state in which elongation is maintained even when the rubber is released from tension. X-ray analysis showed that the crystalline part of the shape-memory NR is not so different from the usual NR. On the other hand, the melting behavior of the shape-memory NR suggested that the contraction force of the stretched amorphous chains decreases the melting temperature of the strain-induced crystals. Based on this idea, the free energy change of the melting of shape memory NR was formulated. The relationship between the factors used in this formulation was compared with the experimental results. A correlation was found between the experimental crystallinity and the crystallinity calculated using the other factors.