Potential competition between fringed-micelle nucleation (N1) and folded-chain nucleation (N2) widely exists in strain-induced crystallization (SIC). However, during uniaxial deformation, no in situ observational evidence of nucleation transition from N2 to N1 in SIC of natural rubber (NR) has been reported yet. In this work, self-seeding provides an effective way for this observation. By the introduction of residual TIC (temperature-induced crystallization)-melting crystallites into pure NR system, in situ synchronic WAXD revealed the formation of low-oriented crystal in the initial deformation stage, which gradually evolves into highly oriented crystal at last. The low-oriented crystal is related to secondary folded-chain nucleation (N2) on the surface of residual TIC-melting crystallites (self-seeding), while newly formed highly oriented crystal is associated with N1. For the first time, the concept of "self-seeding" is innovatively applied to SIC process so that NR exhibits clear nucleation transition phenomenon. Further, theoretical computation of nucleation barrier in the special NR system well reflects that self-seeding has the role of both increasing critical strain of nucleation transition and decreasing onset strain of SIC, thus providing conditions for the observation.