Abstract: In this paper, we proposed a method to determine the nucleation effect of pre-existing crystals on crystallization of the second block in double crystalline block copolymers, which is usually covered by the suppression effect. The nucleation mechanism of poly(ethylene oxide) (PEO) block from the pre-crystallized polyethylene (PE) block in poly(ethylene-co-butene)-b-poly(ethylene glycol) (EmEOn) diblock copolymers was investigated under variable crystallization environments. The crystallization environment for the PEO block was altered by cooling at different cooling rates or successive self-nucleation (SSN) to the PE block. It was found that the presence of nucleation effect is strongly dependent on composition of the block copolymers. The crystallization temperature (Tc) of PEO block in E174EO90 increases as cooling rate applied to the PE block decreases, indicating that PE block can nucleate the crystallization of PEO block and more perfect PE crystals have stronger nucleation effect. In E182EO41 crystallization of the PEO block is confined, shown by the disappearance of self-nucleation domain, and the PE block has no nucleation effect on the crystallization of PEO block. Double crystallization peaks are observed for the PEO block in E182EO41 and the intensity of the crystallization peak at higher temperature increases as the PE crystals become more perfect. After exclusion of homogeneous nucleation mechanism, the higher temperature crystallization peak of the PEO block in E182EO41 is tentatively ascribed to surface nucleation.