Nanoplastics and microplastics (NMPs) in natural environments are an emerging global concern and understanding their formation processes from macro-plastic items during degradation/weathering is critical for predicting their quantities and impacts in different ecological systems. Here, we show the risk of enormous emissions of NMPs from polymer blends, a source that has not been specifically studied, by taking immiscible (most common case) partially biodegradable polymer blends as an example. The blends have the common “sea-island” morphology, where the minor non-biodegradable polymer phase (polyethylene and polypropylene) is dispersed as NMP particles in the major continuous biodegradable matrix (poly(ϵ-caprolactone)). The dispersed NMP particles with spherical and rod-like shapes are gradually liberated and released to the surrounding aquatic environment during the biodegradation of the matrix polymer. Strikingly, the number of released NMPs from the blend is very high. The blend film surface erosion process, induced by enzymatic hydrolysis of the matrix, involving fragmentation, hole formation, and hole wall detachment, was systematically investigated to reveal the NMP release process. Our findings present direct evidence and detailed insights into the high risk of emissions of NMPs from partially biodegradable immiscible polymer blends with a widespread “sea-island” morphology. Efforts from authorities, developers, manufacturers, and the public are needed to avoid the use of non-biodegradable polymers in blends with biodegradable polymers.