The new fluctuation-assisted mechanism for nucleation and crystallization in the isotactic polypropylene/poly(ethylene-co-octene) alloy has been studied. We found that the liquid-liquid phase separation (LLPS) had a dominant, influence on the crystallization kinetics through the nucleation process. After LLPS, the nucleation of crystallization mainly occurred at the interface of the phase-separated domains. It is because that the concentration fluctuations of the LLPS induced the motion of polymer chains and possibly some segmental alignment and/or orientation in the concentration gradient regions through interdiffusion, which could assist the formation of nuclei for crystallization. In other words, the usual nucleation energy barrier could be overcome (or at least partially) by the concentration fluctuation growth of LLPS in the unstable regions. This could be viewed as a new kind of heterogeneous nucleation and could be an addition to the regular nucleation and growth mechanism for crystallization. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 166-172, 2009