In this study a unique phenomenon has been found for isothermal crystallization of double-layer semi-crystalline polymer films. It is surprisingly found that there exists a speeding of poly(L-lactic acid) (PLA) spherulitic growth rate for poly(ethylene oxide)/poly(L-lactic acid) (PEO/PLA) double-layer films at the late stage of isothermal crystallization, which does not exist for PLA/PEO blend films and neat PLA films. The mutual diffusion between PEO and PLA layers plays the key factor to bring out the observed speeding of spherulitic growth rate. This type of study provides an avenue for understanding the interplay between polymer crystallization and interfacial diffusion in multilayer polymer films, which is not available when employing the polymer blend films.