The interplay between the crystallization and mesophase separation of olefin block copolymer (OBC) at different temperatures were investigated by rheology, differential scanning calorimetry (DSC), and polarized optical microscopy (POM). The crystallization behavior at different quench depths were studied based on the concept of liquid-solid transition (LST) and the new suggested suspension model, respectively. The LST method is only applicable under small undercooling, while the suspension model is generally applicable under both small and large undercoolings. It is found that the crystallization behavior of OBC is strongly dependent on the mesophase separation. OBCs with weak mesophase separation exhibits liquid-solid transition at small crystallinity, while those with strong mesophase separation exhibits the opposite trend. Such phenomena explains the different crystallization mode in OBCs.