Biodegradable polymers have been regarded as a promising solution to tackle the pollutions caused by the wide use of conventional polymers. However, during the biodegradation process, the material fragmentation leads to microplastics. In this work, the formation of microplastics from biodegradable poly (butylene adipate-co-terephthalate) (PBAT) in different aquatic environments was investigated and compared with the common non-biodegradable low-density polyethylene (LDPE). The results showed that a much larger quantity of plastic fragments/particles were formed in all aquatic environments from PBAT than from LDPE. In addition, UV-A pretreatment, simulating the exposure to sunlight, increased the rate of PBAT microplastic formation significantly. The size distribution and shapes of the formed microplastics were systematically studied, along with changes in the polymer physicochemical properties such as molecular weight, thermal stability, crystallinity, and mechanical properties, to reveal the formation process of microplastics. This study shows that the microplastic risk from biodegradable polymers is high and needs to be further evaluated with regards to longer timeframes, the biological fate of intermediate products, and final products in freshwater, estuarine and seawater natural habitats. Especially, considering that these microplastics may have good biodegradability in warmer 20 – 25° water but will most likely be highly persistent in the world's cold deep seas. © 2021 The Author(s)