It is widely believed that the extraordinary mechanical properties of natural rubber (NR) are mainly caused by its ability to crystallize at large strains. While several authors believe crystallites working equivalent to nanoscopic filler particles in terms of amplification and filler networking, recent works have identified a crystal-induced strain regulation process as a possibility to explain its outstanding properties. We present a theory that is able to quantitatively describe crystal formation and melting in stretched NR in dependence of temperature and cross-link density. The theory gives reasons for the constant crystals length observed in NR and answers the question why crystallization onset strain is independent of cross-link density. It is tested on the data set of Trabelsi (2003), Albouy (2005), and Rault (2006) reproducing stress-strain data, degree of crystallinity, and crystal sizes at different temperatures using a physically well-defined set of parameters. Additionally, a scheme for NR crystallization involving linear and folded chain crystals is drawn.