h-BN is a widely used ultrathin insulator that can be synthesized in a controllable manner by chemical vapor deposition, similar to the growth of graphene. However, it is challenging to grow large-size single crystalline h-BN because of the ambiguous understanding of its growth mechanism. In this study, we propose a novel in situ formed nanoparticle-assisted growth strategy for large-size single crystalline h-BN growth on conventional polycrystalline copper. We found that the areal nucleation density of h-BN can be suppressed from ∼105 nuclei per mm2 to ∼102 nuclei per mm2 by the in situ formed nanoparticles that were introduced by pre-oxidation. Thus, single crystalline h-BN with lateral length of up to ∼102 μm was readily synthesized. Furthermore, for first time we discovered that the areal nucleation density of h-BN initially decreases and then increases under extreme annealing conditions, indicating that there is a competition-induced limit for suppressing the nucleation of h-BN on copper. This mechanism is universal for h-BN and graphene synthesis, which probably paves the way for large-size graphene/h-BN heterostructures synthesis in the future.