AbstractTwo‐dimensional spherulite growth leads to the encirclement of regions of molten polymer in a polymer film. On further crystallization localized volume defects arise, resulting in thin spots in the film. Since this effect lowers the mechanical strength of films, we call these volume defects “weak spots.” A computer program is developed to evaluate the number, size, and shape of such volume defects for athermal, thermal, and mixed modes of primary nucleation of spherulites. It is shown that the total area of weak spots exceeds 10% of the sample area for all types of nucleation studied. The largest weak spots arise in samples crystallized via athermal and mixed nucleation; their size is of the same order as that of an average spherulite. Formation of weak spots is observed in thin films of poly(ethylene oxide) and poly(methylene oxide). The disadvantageous role of weak spots is confirmed by observation of electric breakdown occurring preferentially in weak spots in polypropylene